TECHNICAL FIELD [0001] The present invention relates to a method for producing monofluoromethane (CLL,F) and relates in more detail to a method for producing monofluoromethane, which is useful as a semiconductor gas, such as etching agent, cleaning agent, etc., and at the same time producing difluoroacetic acid fluoride or its derivatives, which are useful as pharmaceutical and agrochemical intermediates. BACKGROUND OF THE INVENTION [0002] As a method for producing monofluoromethane. there is known a method of subjecting methyl chloride to a fluorination (chlorine-fluorine exchange) on catalyst by hydrogen fluoride (Patent Publication 1). Although this method is high in selectivity, it is low in conversion. Therefore, it is difficult to say that this method is an efficient production method. Furthermore, the target product, monofluoromethane (boiling point: -78°C), and a by-product of hydrogen chloride (boiling point: -85°C) not only have boiling points close to each other, but also show an azeotropic phenomenon. Therefore, a distillation separation is not easy, and a complicated purification process is used (Patent Publication 2). Furthermore, there is known a method of fluorinating methyl iodide using a tetra-n- butylammonium salt (Non-patent Publication 1), but the starting raw material is not easily available as compared with the method of Patent Publication 1. Furthermore, the starting raw materials of these methods are not only high in toxicity, but also are ozone depleting materials. Therefore, attention is necessary in handling. Furthermore, as described in Patent Publication 2, when the product is contaminated with chlorine, bromine, iodine, etc., which are high in reactivity with radicals, there occurs an influence on the etching rate, etc. Therefore, it is not preferable to use these fluorine-containing materials as the raw material. [0003] It is described in Patent Publication 3 that an alkyl fluoride and the alkyl fluoride's degradation products, that is, olefins and hydrogen fluoride, are formed as by-products, when synthesizing difluoroacetic acid fluoride or a difluoroacetic acid ester by bringing l-alkoxy-l,l,2,2-tetrafluoroethane into contact with a metal oxide catalyst, but there is no disclosure therein regarding monofluoromethane's yield, purity, isolation purification method, application method, etc. PRIOR ART PUBLICATIONS PATENT PUBLICATIONS [0004J Patent Publication 1: International Publication 2005/026090 Pamphlet. Patent Publication 2: Japanese Patent Application Publication 2006-111611. Patent Publication 3: Japanese Patent Application Publication Heisei 8-92162. NON-PATENT PUBLICATIONS [0005] Non-patent Publication 1: J. Am. Chem. Soc, 127(7), 2050-2051 (2005). SUMMARY OF THE INVENTION [0006] It is known that, due to the raw material, a fluorine-containing, semiconductor gas produced by a halogen-fluorine exchange reaction frequently contains as impurities halogens other than fluorine, such as chlorine, bromine, iodine, etc., which are avoided in semiconductor device production steps, thereby generating various problems in a precision etching, such as anisotropic etching. Thus, the present invention provides a method for practically and efficiently producing monofluoromethane that is substantially free from halogens other than fluorine. [0007] As a result of a study of the monofluoromethane production method, the present inventors have found that monofluoromethane of high purity can easily be isolated with high yield from a pyrolysis product formed by pyrolyzing 1-methoxy- 1,1,2,2-tetrafluoroethane while it is brought into contact with a catalyst. [0008] That is, the present invention is as follows. [0009] [Invention 1] A method for producing monofluoromethane, comprising at least a pyrolysis step in which l-methoxy-l,l,2,2-tetrafluoroethane is pyrolyzed while it is brought into contact with a catalyst, and a step in which monofluoromethane is collected from a pyrolysis product. [0010] [Invention 2] Invention 1 in which the step of collecting monofluoromethane is a step comprising a step of separating monofluoromethane by liquefying a part of the pyrolysis product. [0011] [Invention 31 Invention 2 in which the liquefaction of a part of the pyrolysis product is conducted by cooling. [0012] [Invention 41 Invention 3 in which the cooling temperature is from -80 to -5°C. |0013] [Invention 51 Invention 1 in which the step of collecting monofluoromethane is a step comprising a step of absorbing difluoroacetic acid fluoride into a solvent that is inert against difluoroacetic acid fluoride. [00141 [Invention 6] Invention 5 in which the solvent that is inert against difluoroacetic acid fluoride is a hydrocarbon compound. [0015] [Invention 7] Invention 1 in which the step of collecting monofluoromethane is a step comprising a step of bringing into contact with a compound that is active to difluoroacetic acid fluoride. [0016] [Invention 8] Invention 7 in which the compound that is active to difluoroacetic acid fluoride is water, an alcohol, primary amine, secondary amine, or α,β-unsaturated carboxylic acid ester. [0017] [Invention 9] Invention 7 or Invention 8, in which a solvent is made present in the step of bringing into contact with the compound that is active to difluoroacetic acid fluoride. [0018] [Invention 101 Inventions 7-9. in which a basic substance is made present in the step of bringing into contact with the compound that is active to difluoroacetic acid fluoride. [0019] [Invention 11] Inventions 1-10. in which the pyrolysis step is conducted such that a metal oxide, a partially fluorinated metal oxide, a metal fluoride, an untreated or fluorinated phosphoric acid, or an untreated or fluorinated phosphate is used as the catalyst and that the pyrolysis temperature is made to be from 100°C to 400°C. [0020] [Invention 12] Inventions 1 to 10, in which the pyrolysis step is conducted such that alumina, a partially fluorinated alumina, or aluminum fluoride is used as the catalyst and that the pyrolysis temperature is made to be from 130°C to 260°C. [0021] [Invention 13] An etching agent or cleaning gas in a semiconductor device production step, which is characterized by comprising monofluoromethane produced by the method according to any one of claims 1-12. [0022] [Invention 14] An etching method or cleaning method in a semiconductor device production step, which is characterized by using monofluoromethane produced by the method according to any one of claims 1-12. [0023] [Invention 15] Inventions 1 to 6, comprising a step of obtaining difluoroacetic acid fluoride by separating the same, with the obtainment of monofluoromethane. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS [0024] Fig. 1A is a sectional schematic view of an etching sample used in Monofluoromethane Use Examples 1 and 2. Fig. 1B is a sectional schematic view of an etching sample used in Monofluoromethane Use Examples 1 and 2, after etching. Fig. 2 is a schematic sectional view of a remote plasma device used in Monofluoromethane Use Examples 1 and 2. Fig. 3 is a schematic view of an apparatus used in Examples 1-21. Fig. 4 is a schematic view of an apparatus used in Examples 22-24 and Reference Examples 5 and 6. Fig. 5 is a schematic view of an apparatus used in Example 26. Fig. 6 is a schematic view of an apparatus used in Examples 27 and 28. DETAILED DESCRIPTION [0025J Since the raw materials do not contain chlorine, etc. in the production method of the present invention, it is possible to produce monofluoromethane of high purity containing no halogens except fluorine as impurities. In the method of the present invention, it is possible to produce a monofluoromethane of high purity, which is usable as an etching agent or cleaning agent in semiconductor industries, without conducting a purification operation by a complicated means. The production method of the present invention is a practical production method, since there is used as the raw material l-methoxy-l,l,2,2-tetrafluoroethane, which is small in terms of the burden on the global environment, such as ozone layer depletion, and is low in toxicity. Furthermore, since difluoroacetic acid fluoride, etc., which are obtained as by-products, have uses as pharmaceutical and agrochemical intermediates, it is possible to effectively use the raw material. [0026] The production method of the present invention is a method in which 1- methoxy-l,l,2,2-tetrafluoroethane is pyrolyzed in the presence of a catalyst to obtain a pyrolysis product containing monofluoromethane, and monofluoromethane is separated from this pyrolysis product to produce the same. The reaction, in which this method is involved, is represented by the following formula. CHF2CF2OC113 → CH3F + CHF2COF [0027] The raw material of the present invention, l-methoxy-1,1,2,2- tetrafluoroethane, can be obtained by a publicly-known production method. For example, 1 -methoxy-1.1,2,2-tetrafluoroethane can be synthesized by a method in which methanol and tetrafluoroethylene are reacted in the presence of potassium hydroxide (J. Am. Chem. Soc, 73, 1329 (1951)). [0028] The pyrolysis catalyst according to the present invention is a metal oxide, a partially fluorinated metal oxide, a metal fluoride, a phosphoric acid, or a phosphate, and is used as a solid catalyst. [0029J The metal oxide can be exemplified by alumina, titania, zirconia, etc. Alumina, which is easily available, is particularly preferable. As to the alumina, it is possible to precipitate aluminum hydroxide by adding ammonia to an aqueous solution of an aluminum salt, such as aluminum sulfate, aluminum nitrate, etc., followed by shaping and drying to make a shaped product having an arbitrary size and shape. As the crystal form, γ-alumina, which is large in specific surface area, is preferable. It is possible to use α-alumina and y-alumina. which are on the market as desiccant, adsorbent, catalyst support, etc. As to the metal oxide, prior to use, it is preferable to conduct a fluorination treatment by partially replacing oxygen atoms with fluorine atoms by hydrogen fluoride, an organic fluorine compound gas, etc. to make a partially fluorinated metal oxide to prevent the catalyst from being fluorinated and lowering in activity during the reaction. In the case of not conducting this lluorination treatment, once monofluoromethane produced by the pyrolysis and the raw material are brought into contact with the metal oxide at the reaction temperature, the catalyst may be fluorinated to make the catalyst activity unstable, and monofluoromethane and difluoroacetic acid fluoride may be decomposed, thereby increasing hydrocarbon by-products such as methane. For the fluorination treatment, hydrogen fluoride is preferable, since it has a low price as the fluorination agent and does not cause precipitation of carbon by the treatment. [0030] Of the metal fluorides, particularly aluminum fluoride (A1F3) or calcium fluoride (CaF2) is preferable. It is preferable that these fluorides are in the form of anhydride. In the case of preparing them from aqueous materials, it is preferable to conduct a dehydration treatment by heating. In these catalysts, metal is fully fluorinated. Therefore, there does not occur a phenomenon similar to the case of metal oxides that the catalyst takes fluorine out of the raw material or the product. Even in metal fluorides, however, a fluorination treatment by hydrogen fluoride, etc. is preferable, since it activates the catalyst surface. [0031] As the catalyst of the pyrolysis, a phosphoric acid or phosphate (in the present specification, both of phosphoric acid and phosphate may be referred to as "phosphate") is also preferable. The phosphate may be one supported on a support. As the phosphoric acid, any of orthophosphoric acid, polyphosphoric acid, and metaphosphoric acid will do. As the polyphosphoric acid, it is possible to mention pyrophosphoric acid. etc. The phosphates are metal sails of these phosphoric acids. Due to easiness in handling, one being orthophosphoric acid is preferable. [0032] The phosphate is not particularly limited. It is possible to mention phosphates of at least one metal selected from the group consisting of hydrogen, aluminum, boron, alkali-earth metals, titanium, zirconium, lanthanum, cerium, yttrium, rare-earth metals, vanadium, niobium, chromium, manganese, iron, cobalt, and nickel. Preferably, the phosphates as a major component are aluminum phosphate, cerium phosphate, boron phosphate, titanium phosphate, zirconium phosphate, chromium phosphate, etc. It is also preferable that these contain other metals. Specifically, preferable ones are cerium, lanthanum, yttrium, chromium, iron, cobalt, nickel, etc. Cerium, iron, and yttrium are more preferable. Of these, more preferable ones are aluminum phosphate, cerium phosphate, and a phosphate formed of these two. [0033] The method for preparing the phosphate catalyst is not particularly limited. A commercial phosphate may be used as it is. It may be prepared by a general precipitation method. As a specific preparation method of the precipitation method, for example, to a mixed aqueous solution of a metal nitrate (in the case of a plurality of metals, a solution of each salt is prepared) and a phosphoric acid, a diluted ammonia water is added dropwise to adjust pH to make a precipitation. According to need, it is allowed to stand for aging. Then, it is washed with water. Then, it is checked by conductivity of the washing water or the like that the water washing has been sufficient. In some cases, it is checked by sampling a part of the slurry and measuring the cation contained therein. Then, it is filtered and dried. The temperature for drying is not particularly limited. It is preferably 80°C to 150°C, more preferably 100°C to 130°C. The obtained dried matter is pulverized to make the particle size even, or further pulverized into a pellet or spherical shape. Then, it is baked in air or nitrogen atmosphere in a condition of 200°C to 1500°C. The baking is conducted at preferably 400 to 1300°C, more preferably 500°C to 900°C. [0034] Although it also depends on temperature, the baking time is about 1 hour to 50 hours, preferably about 2 hours to 24 hours. The baking treatment is a treatment necessary for stabilizing the phosphate. Therefore, in case that it is treated at a temperature lower than the temperature of the pyrolysis reaction or that the treatment time is short, it may not sufficiently show the catalytic activity at the initial stage of the reaction. Furthermore, it is not preferable to conduct the baking treatment at a temperature higher than the above temperature range or for a long time, since not only it requires an excessive heating energy, but also it may cause crystallization of the catalyst to spoil catalytic activity. [0035] It is preferable to conduct the operation of adding a metal component other than a main component by using a metal salt. It is used as a nitrate, chloride, oxide, phosphate, or the like of the metal. Of these, nitrate is preferable due to its high solubility in water. The amount of addition is not particularly limited. In general, it is 1 gram atom or less, preferably 0.5 gram atoms or less, more preferably 0.3 gram atoms or less, relative to 1 gram atom of phosphorus. These metal components may be added to a metal salt solution prior to the precipitation when preparing the catalyst. Alternatively, it may be conducted by an immersion of the phosphate catalyst after the catalyst baking into a metal salt solution, or the like. [0036] It is also possible to use the catalyst of a metal oxide, a partially fluorinated metal oxide, a metal fluoride, a phosphoric acid or a phosphate as it is in the form of powder as a catalyst for a fluidized bed. It can also be used as a catalyst for a fixed bed by pellet compression. When the powder is compressed into pellets, a binder may be added. As to the binder, it is possible to use saccharides, polymer compounds, metal oxides, etc., which have conventionally generally been used. By adding a small amount of a phosphoric acid, such as orthophosphoric acid, polyphosphoric acid, metaphosphoric acid, etc., the pellet compression can efficiently be conducted without spoiling the catalytic activity. [0037] As the catalyst, an active component as mentioned above can also be used as it is. However, the use in a condition supported on a support is preferable. As the support, it is possible to mention metal oxides, such as alumina, titania, zirconia, zirconium oxide sulfate (ZrO(SO4)), etc., silicon carbides, silicon nitrides, activated carbon, etc. Activated carbon is particularly preferable. [0038] A phosphoric acid-supported catalyst can be prepared by an immersion of the support into a phosphoric acid solution for an impregnation or by a spraying to make a covered or adsorbed one, followed by drying. In the case of supporting a phosphate, it can be prepared by an impregnation with a single solution of at least one compound to be supported, or by a spraying for covering or adsorption, followed by drying. Furthermore, it is also possible to conduct an impregnation or the like with a solution of a first compound, followed by drying and then an impregnation or the like with a solution of another different compound. Furthermore, it is also possible to prepare a phosphate-supported catalyst by conducting a preparation method by the above-mentioned phosphate precipitation method in the presence of a support such as activated carbon, etc. [0039] The activated carbon can be any of vegetable series using raw materials such as wood, charcoal, coconut husk coal, palm core coal, and raw ash; coal series using raw materials such as peat, lignite, brown coal, bituminous coal, and anthracite; petroleum series using raw materials such as petroleum residue and oil carbon; synthetic resin series using raw materials such as carbonated polyvinylidene chloride. It is possible to use one by selecting from these commercial activated carbons. For example, it is possible to cite an activated carbon (BPL GRANULAR ACTIVATED CARBON made by TOYO CALGON CO.) produced from bituminous coal, a coconut husk coal (GRANULAR SH1RO SAGI GX, SX, CX and XRC made by Japan BnviroChemicals, Ltd. and PCB made by TOYO CALGON CO.). etc., but it is not limited to these. It is used generally in the form of granules in terms of shape and size, too. It is possible to use one in the form of sphere, fiber, powder, honeycomb, or the like in an ordinary knowledge scope as long as it fits into the reactor. [0040] The activated carbon used in the present invention is preferably an activated carbon that is large in specific surface area. It is acceptable that the specific surface area of the activated carbon is in a range of the standard of commercial products. Each of them is 400m2/g to 3000m2/g, preferably 800nr/g to 2000m2/g. Furthermore, in the case of using the activated carbon as a support, it is optional to previously conduct an activation of the support surface and removal of ash by immersing it in a basic aqueous solution, such as ammonium hydroxide, sodium hydroxide, potassium hydroxide, etc., at around ordinal}' temperature for a period of time of about 10 hours or longer or by conducting a pretreatment by an acid, such as nitric acid, hydrochloric acid, hydrofluoric acid, etc., which is generally conducted upon using activated carbon as a catalyst support. [0041] As to also a phosphate catalyst or a phosphate-supported catalyst of the present invention, it is effective for the catalyst lifetime prolongation and the abnormal reaction prevention to conduct a fluorination treatment by previously bringing it into contact, prior to use, with a fluorine-containing compound such as hydrogen fluoride, a fluorinated hydrocarbon or a fluorochlorinated hydrocarbon or the like, since it prevents the compositional change of the catalyst during the reaction. [0042] As a support of a metal oxide or the like of the present invention, it may contain other atoms other than the metal component and oxygen. At least one metal oxide selected from the group consisting of alumina (Al2O3), zirconia (ZrO2), titania (TiO2). zirconium oxide sulfate, and partially fluorinated oxides of these is preferable. Alumina and a partially fluorinated alumina are particularly preferable in terms of catalytic activity and catalyst lifetime. The proportions of oxygen atoms and fluorine atoms in the catalyst are not particularly limited. [0043] In the present specification and the claims, unless particularly limited, oxides, such as alumina and zirconia, subjected to a partial fluorination, chlorination or the like as mentioned above, may be denoted by oxide names such as "alumina" and "zirconia"'. [0044] The fluorination treatment by hydrogen fluoride can greatly increase activity of the reaction. It is preferable to conduct that by bringing into contact with hydrogen fluoride at a temperature higher than at least the temperature of the pyrolysis. Specifically, in the case of a metal oxide such as alumina or a metal fluoride such as aluminum fluoride, it is about 200-600°C, preferably about 250- 500°C, more preferably 300-400°C. In the case of a phosphate alone, it is about 200-700°C. preferably about 250-600°C, more preferably 300-550°C. On the other hand, in the case of a phosphate-supported catalyst, it is about 200-600°C, preferably about 250-500°C, more preferably 300-400°C. In any of them, time is necessary for the treatment at a temperature lower than 200°C. It is not preferable to conduct the treatment to exceed the maximum temperature range, since an excessive heating energy is necessary. Furthermore, the treatment time cannot be limited, since it also relates to the amount of treatment and the treatment temperature. It is about 1 hour to 10 days, preferably about 3 hours to 7 days. [0045] In the pyrolysis. of the above-explained catalysts, aluminum fluoride, calcium fluoride, and a catalyst prepared by treating alumina or aluminum phosphate by hydrogen fluoride are particularly preferable. [0046] For the pyrolysis, it is possible to mention a continuous gas-phase flow mode as the most preferable mode. It is, however, not limited to this. The size and the shape of the reactor can suitably be changed, depending on the amounts of the reactants. etc. [0047] In the pyrolysis. it is also possible to make an inert gas exist in the reaction conditions, but there becomes troublesome an operation to separate monofluoromethane and the inert gas. [0048] The pyrolysis lemperature depends on the type of the catalyst or the contact time. Normally, it is 100-400°C, preferably 110-350°C. more preferably 130- 320°C, still more preferably 130-260°C, particularly preferably 140-200°C. At a reaction temperature lower than 100°C, selectivity of monofluoromethane is high, but productivity is low due to low conversion. Therefore, it is not preferable. At a reaction temperature exceeding 400°C, conversion is almost 100%. It becomes, however, necessary to have a heat resistance severe on the reaction apparatus and an excessive heating energy. Therefore, it is economical!} not preferable and may cause side reactions. For example, as shown in the after-mentioned Reference Example 5, when difluoroacetic acid fluoride formed is brought into contact with the catalyst at a high temperature, it may decompose into (rilluoromethane (CHF3). This CHF3 (boiling point = -82°C) is close to the target compound, monofluoromethane (boiling point -78°C), in terms of boiling point, thereby imposing a burden on the distillation separation. Therefore, it is desirable to suppress the production as much as possible. [0049] The reaction time (contact time) depends on the reaction temperature. Normally, it is 0.1-1000 seconds, preferably 1-500 seconds, more preferably 10-300 seconds. In case that the reaction time is shorter than 0.1 seconds, there is a risk that conversion lowers. On the other hand, if it is longer than 1000 seconds, productivity lowers. Therefore, each of them is not preferable. On the contrary, even in a region where the reaction is very slow with a reaction temperature lower than 100°C, it is also possible to improve conversion by extending the contact time. [0050] The reaction pressure is not particularly limited. Any of ordinary pressure, reduced pressure and pressurization will do. Around 0.05-0.5 MPa (0.5-5 atmospheres) is preferable. Normally, a pressure around atmospheric pressure, at which operation is easy, is preferable. [0051] The pyrolysis reaction makes it possible to have a conversion of 1-methoxy- 1,1.2,2-tetrafluoroethane that is substantially 100%. Conversion is correlated with the percentage of trifluoromethane produced as a by-product. Therefore, if it is desired to decrease the production of trifluoromethane and simplify the purification step, it is preferable to make it 30-95%, more preferably 50-90%. If conversion is less than 30%, productivity of monofluoromethane is low. If it exceeds 95%, the production of trifluoromethane as a by-product may increase. [0052] In the catalyst of the pyrolysis reaction, coking may occur over time, and the catalyst activity may lower. By bringing the catalyst having a lowered activity into contact with oxygen (oxygen treatment) at 200°C to 1200°C, preferably 400°C to 800°C, it is possible to easily recover the activity. It is easy to conduct the oxygen treatment by allowing oxygen to flow in a condition that the reaction tube is charged with the catalyst or thai an external apparatus is charged with the catalyst. In the flow of oxygen, another gas may be coexistent. It is possible to use oxygen, air, a nitrogen-diluted oxygen, etc. A nitrogen-diluted air or air is economically preferable. Furthermore, it is also possible to use an oxidative gas such as chlorine, fluorine, etc. Furthermore, it is preferable to bring hydrogen fluoride into contact therewith after conducting these treatments, since the catalyst surface is further activated. [0053] Main components of the pyrolysis product generated by the pyrolysis are monofluoromethane and difluoroacetic acid fluoride. It may contain, besides the unreacted l-methoxv-l.l,2,2-tetrafluoroethane (HFE-254pc), small amounts of methane (CH4), ethylene (C2H4), trifluoromethane (CHF.O, propylene (C3H6), methyl difluoroacctate (CHF2COOCH3), difluoroacetic acid (CHF2COOH), etc. The method of separating and taking monofluoromethane from the pyrolysis product is not limited. Specifically, there are a distillation separation method to use the difference in boiling point of monofluoromethane and other components, an absorption separation method to use the difference in solubility in solvent, or a reaction separation method to conduct a separation after reacting difluoroacetic acid fluoride with a compound having an active hydrogen atom. [0054] [Distillation Separation Method] There are large differences in boiling point between the target compound monofluoromethane (boiling point: -78°C) and other major components, that is, difluoroacetic acid fluoride (boiling point: 0°C) and the unreacted HFE-254pc (boiling point: 40°C). Therefore, it is possible to easily separate and recover a component mainly containing monofluoromethane by a simple cooling liquefaction, when cooling the pyrolysis product (gas) flown out of the pyrolysis apparatus. Naturally, it is also possible to liquefy the pyrolysis product by pressurization. Even in this case, it is preferable to conduct cooling. Upon this, some components are liquefied, and it is possible to easily separate it into a composition mainly containing monofluoromethane as a low-boiling-point component and difluoroacetic acid fluoride or a mixture of difluoroacetic acid fluoride and I lFE-254pc as a high- boiling-point component. The composition can arbitrarily be varied by the cooling temperature. Normally, the low-boiling-point component may contain CH4, C2H4, CHF3. C3H6, etc. as impurities, and the high-boiling-point component may similarly contain CHF2COOCH3, CHF2COOH, etc. [0055] The cooling temperature depends on the operation pressure, the amount of gas flow, the cooling capacity, etc. The cooling temperature under a pressurized condition can easily be estimated from the following explanation and the data of vapor pressure. Under atmospheric pressure, it suffices to make that -80 to —5°C, preferably -78 to -20°C. Monofluoromethane is not substantially condensed (liquefied) at -78°C. and it is also possible to use a cooling medium cooled with carbon dioxide gas or solid carbonic acid (dry ice). The cooling method is not particularly limited, and a publicly-known means can be applied. For example, it is possible to mention a method by a condenser having a normal multi-pipe structure, and a method in which gas is allowed to flow through a void tower of which outside is cooled with a cooling medium or the like and a packed tower having in its inside a distillation packing member, and like methods. [0056] In place of a simple cooling liquefaction, it is also possible to conduct a distillation separation on the pyrolysis product using a rectifying column. As the rectifying column, it is possible to mention packed column, bubble cap column, etc. As to the distillation apparatus and the distillation method, it is possible to follow publicly-known apparatus and method. The condition of the distillation can be set in accordance with a composition of the target low-boiling-point component or high- boiling-point component. In order to enlarge the composition of monofluoromethane as a low-boiling-point component, it is preferable to conduct a distillation in which the column top is adjusted to around -78°C, and the column bottom is adjusted to around 0 to 50°C. In that case, the low-boiling-point component may contain small amounts of CH4, C2H4, CHF3, C3H6, etc. In the case of using a rectifying column, the low-boiling-point component which has been distilled out and consists essentially of monofluoromethane has a sufficiently high purity and can be made into a semiconductor gas product. The high-boiling-point component to be taken out of the column bottom contains difluoroacetic acid fluoride and the unreacled HFE-254pc as main components. The high-boiling-point component is further separated by distillation into difluoroacetic acid fluoride and HFE-254pc. Difluoroacetic acid fluoride can be used as a synthesis raw material of various reactions, and HFE-254pc can be used as a recycled raw material for the pyrolysis step. [0057J [Absorption Separation Method] The pyrolysis product formed by the pyrolysis is brought into contact with an inert solvent that does not react with difluoroacetic acid lluoride (hereinafter referred to as the inert solvent). With this, difluoroacetic acid fluoride contained in the pyrolysis product is absorbed into the solvent, and thereby it is possible to take the undissolved monofluoromethane out. [0058] Herein, the inert solvent is a solvent that is in the form of liquid upon the contact for the absorption and does not have active hydrogen atoms. Furthermore, a solvent that is free from halogen atoms other than fluorine, such as chlorine, is preferable. As such solvent, it is possible to mention specifically aliphatic or aromatic hydrocarbon compounds, ketones, ethers, esters, etc. As the aliphatic hydrocarbon compounds, C5-20 hydrocarbon compounds are preferable. It is possible to mention pentane, hexane, heptane, octane, nonane, decane, undecane, cyclopentane, cyclohcxane, cycloheptane, methylcyclohexane, etc. As the aromatic hydrocarbon compounds, C6-20 aromatic compounds are preferable. It is possible to mention benzene, toluene, o-, m- or p-xylene, mesitylene, ethylbenzene, fluorobenzene, 0-. m- or p-trifluoromethylbenzene, bistrifluoromethylbenzene, etc. As the ketones, it is possible to mention acetone, methyl ethyl ketone, diethyl ketone, methyl isobulyl ketone, etc. As the ethers, it is possible to mention dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, etc. As the esters, it is possible to mention methyl formate, ethyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl butyrate, ethyl butyrate, butyl butyrate, etc. Even if alkyl groups contained in these compounds are isomers, they can similarly be used. [0059] The contact between the pyrolysis product and the inert solvent is conducted at -70 to +20°C, preferably -30 to 0°C. Exceeding +20°C is not preferable, since solubility of difluoroacetic acid fluoride in the inert solvent lowers, thereby lowering the absorption efficiency. Furthermore, at a temperature lower than -70°C, the inert solvent increases in viscosity and may be solidified. Furthermore, monofluoromethane may also be absorbed to lower yield. Therefore, it is not preferable. This contact can also be conducted under pressurization. In that case, the contact temperature is also different. Normally, it is preferable to conduct that at around atmospheric pressure in terms of apparatus and operation. [0060] The method of contact between the pyrolysis product and the inert solvent is not limited. It is possible to use a publicly-known gas-liquid contact method. For example, it is possible to mention methods using packed column, tray column, spray column, scrubber, wet wall column, bubble column, three-phase fluidized bed, bubble-stirred tank, etc. Of these, packed column, spray column, bubble column, bubble-stirred tank. etc. are preferable. [0061 ] In the packed column, the pyrolysis product is fed from below, and the inert solvent is fed by circulation with a liquid distribution plate at a high position. On the packing material surface, the pyrolysis product is absorbed into the inert solvent and is retained as an absorption-completed solvent in a tank provided at the bottom of the packed column or outside thereof. Monofluoromethane not absorbed is discharged from the top of the packed column. [0062] In the spray column, the inert solvent is distributed in the column from the column top of a void column through a spray nozzle in the form of many fine liquid droplets. The pyrolysis fed from the column bottom is raised. In the column, difluoroacetic acid fluoride is absorbed into the inert solvent, and monofluoromethane not absorbed is discharged from the top of the packed column. [0063] In the bubble column or bubble-stirred tank, the pyrolysis product is bubbled into a container charged with the inert solvent, from a liquid bottom. As bubbles go up, difluoroacetic acid fluoride contained in the pyrolysis product is absorbed into the inert solvent, and monofluoromethane not absorbed is discharged from the top of the packed column, in the bubble column, a sparger is used for bubbling the pyrolysis product into the column. Alternatively, it is optional to use a publicly- known method for gaining the residence time of the pyrolysis product as bubbles in the column. In the bubble-stirred tank, it is possible to increase the contact efficiency by bubbling the pyrolysis product into the tank and making the bubbles smaller by a baffle plate and a stirring blade. [0064] In these contact methods, it is also possible to use the same apparatuses or different apparatuses b\ combining them in series. In the case of the bubbling method using the bubbling column or bubble-stirred tank (Both of them may simply be referred to as "tank".), there is preferable a multi-tank form in which a plurality of tanks are connected in series. As an example, the case of an absorbing tank group formed of three tanks is explained. Firstly, piping is set in a manner that the pyrolysis product is introduced into the first tank and that a component mainly containing monofluoromethane is recovered through the second tank. When the inert solvent of the first tank is saturated with difluoroacetic acid fluoride, piping is changed in a manner to switch the target of introducing the pyrolysis product to the second tank and recover a component mainly containing monofluoromethane through the third tank. When the inert solvent of the second tank is saturated with difluoroacetic acid fluoride, piping is changed in a manner that the third tank is made to be the target of introducing the pyrolysis product and that the first tank prepared by taking the absorption-completed solvent out and charging the same with a newly-prepared inert solvent is made to be an exit of a component mainly containing monofluoromethane. Hereinafter, it can be conducted similarly. [0065] Normally, a low-boiling-point component obtained by the absorption separation method contains a solvent used as the absorbing liquid. It is desirable to remove the solvent b\ distillation. It can easily be removed by a simple distillation or the after-mentioned fractional distillation. [0066] The absorbing liquid (absorption-completed solvent) used in the absorption separation method contains as major components the inert solvent, in addition to difluoroacetic acid fluoride and the unreacted HFE-254pc. Furthermore, the absorption-completed solvent is separated by distillation into difluoroacetic acid fluoride and HFE-254pc. Difluoroacetic acid fluoride can be used as a synthesis raw material of various reactions, and HFE-254pc can be used as a recycled raw material for the pyrolysis step. [0067] [Reaction Separation Method] In the reaction separation method, difluoroacetie acid fluoride contained in the reaction product generated by the pyrolysis is converted by a reaction into a stable compound having a high boiling point, followed by separation from monofluoromcthane. It is also possible to simultaneously conduct the conversion and the separation in the same container or in different containers. As an active compound that becomes a reaction partner (reaction reagent), it is possible to mention compounds with an active hydrogen atom, such as water, alcohols, or primary amines or secondary amines, a,p-unsaturated carboxylic acid esters, etc. It is, however, not limited to these. Of these, water or alcohols are preferable. The reactions of these compounds can be exemplified by the following formulas. [0068] CHF2COF + H2O → CHF2COOH + HF CHF2COF + ROH → CHF2COOR + HF CHF2COF + RNH2 → CHF2CONRH + HF CHF2COF + R2NH → CHF2CONR2 + HF [0069] In the reaction formulas, R represents an organic group. In these reactions, it is preferable to make a basic substance present, as a catalyst or as an acid acceptor for stabilizing hydrogen fluoride (HF) formed. As the basic substance, alkali metal hydroxides or carbonates, such as sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate, and tertiary amines are preferable. In the case of using water as the reaction reagent, potassium is particularly preferable as the alkali metal. If the basic substance is made to be present, difluoroacetie acid is converted to a difluoroacetate. [0070] The alcohols (ROH) are not particularly limited, As R, it is possible to mention a C1-8, alkyl group or fluorine-containing alkyl group, which is optionally branched, a cycloalkyl group, an aryl group, and an aralkyl group, which optionally have an alkyl group as a substituent. Of these, C1-8 alkyl group or a C2-8 fluorine- containing alkyl group is preferable. Furthermore, C1-4 alkyl group or a C2-4 fluorinated alkyl group is more preferable. The alcohols may be polyhydric alcohols. The C1-8 alkyl group can be exemplified by methyl group, ethyl group, n- propyl group, isopropyl group, n-butyl group, s-butyl group, t-butyl group, n-pentyl group, and isopentyl group. The C2-8 fluorine-containing alkyl group can be exemplified by 2,2,2-trifluoroethyl group, pentafluoroethyl group, 2,2,3,3,3- pentafluoropropyl group, n-hexafluoropropyl group, hexafluoroisopropyl group, etc. As the polyhydric alcohols, one having a valence of 2-5 and a carbon number of 1-8 is preferable, and one having a carbon number of 1-4 is more preferable. Specifically, it is possible to mention glycol, ethylene glycol, diethylene glycol, tricthylcnc glycol, propylene glycol, dipropylene glycol, pentaerythritol, etc. [0071] The alcohols can also be used as metal alkoxides of the alcohols. As the metal, it is possible to mention sodium, potassium, lithium, etc. A sodium or potassium alkoxide of a C1.4 alcohol is preferable. Specifically, it is possible to mention sodium methoxide, sodium elhoxide, sodium propoxide, potassium butoxide. potassium methoxide, potassium ethoxide, potassium propoxide, potassium butoxide. and ones in which these alkyl groups are isomers. [0072] As the primary amine and the secondary amine, there is preferable an amine represented by the general formula R1 R2 NH (R1 and R2 are a hydrogen or a straight- chain, branched or cyclic alkyl group, and the total number of carbons is 3-15.). If the total number of carbons is less than 3, the contact must be conducted at low temperatures due to its low boiling point. This is not preferable, due to a danger of the contamination of the low-boiling-point component. [0073] As an amine that is represented by the above general formula and has a total carbon number of 3-15, it is possible to mention diethylamine, dipropylamine, diisopropylamine, dibutylamine, diamylamine, propylamine, isopropylamine, butylamine, amylaminc, hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, etc. Of these, diethylamine, dipropylamine, diisopropylamine, dibutylamine, etc., which are easily available, are more preferable, and diethylamine is particularly preferable. These amines can be used as a mixture, too. [0074] As the α,β-imsaturated carboxylic acid ester, an acrylate or methacrylate is preferable. It can be exemplified by methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylatc, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, etc. [0075J The compounds having an active hydrogen atom, such as water, alcohols, primary amines, secondary amines, etc., may he mixed with each other. It is used in an excessive amount relative to difluoroacetic acid fluoride. Furthermore, these reagents may be used together with an inert solvent. Its use is preferable in the case of ensuring fluidity for a solid or a low-melting-point compound as the reagent. Although the usage of the solvent is not limited, it is 30-10000 parts by mass, preferably 100-1000 parts by mass, relative to 100 parts by mass of the reagent. As the inert solvent, it is possible to mention inert solvents described in the Absorption Separation Method. [0076] The temperature for the contact is not particularly limited. Therefore, it may be a condition without conducting a particular heating or cooling. Normally, it may be about 0-50°C. The pressure does not have a particular effect on the reaction. Therefore, it may be conducted under pressurization or under reduced pressure. It suffices to conduct that at around atmospheric pressure, at which compression or decompression is not particularly conducted. [0077] The tertiary amine as a basic substance made to exist upon the contact is not particularly limited. It is preferably a tertiary amine represented by the general formula R1R2 R3N (R1, R2 and R3 are straight-chain, branched or cyclic alkyl groups, and the total number of carbons is 6-15.). Even a tertiary amine having a total carbon number of 5 or less can capture hydrogen fluoride in the form of a tertiary amine/hydrogen fluoride salt. Since such tertiary amine is highly soluble in water, it lowers the recovery in case that the tertiary amine is recovered by decomposing the tertiary aminc/hydrogen fluoride salt by water or an aqueous solution to separate the tertiary amine from the aqueous layer. This increases the amount of waste material. Thus, it is not preferable. Since a tertiary amine having a total carbon number of 16 or greater is less soluble in water, it is suitable for the decomposition and the recovery with water. It is, however, small in terms of the amount of hydrogen fluoride captured per its weight. Therefore, it is practically not preferable. [0078] As the tertiary amine represented by the above general formula and having a total carbon number of 6-15, it is possible to mention tri-n-propylamine, tri- isopropylamine. tri-n-butylamine, tri-isobutylamine, tri-sec-butylamine, tri-tert- butylamine, tri-n-amylamine, tri-isoamylamine, tri-sec-amylamine, tri-tert- amylamine, N-methyldi-n-butylamine, N-methyldiisobutylamine, N-methyldi-tert- butylamine, N.N-diisopropylbutylamine, N,N-dimethyl-n-octylamine, N,N- dimethylnonylamine, RN-dimethyldecylamine, N,N-dimethylundecylamine, N,N- dimethyldodecylamine, N-methyldihexylamine, etc. Of these, tri-n-propylamine, tri-isopropylamine, tri-n-butylamine, tri-isobutylamine, tri-n-amylamine, tri- isoamylamine, etc. are more preferable. Tri-n-butylamine is particularly preferable. These tertiary amines can also be used as a mixture. [0079] The tertiary amine is added by 0.5 mols or greater relative to 1 mol of difluoroacetic acid fluoride. In the case of using the tertiary amine in excess, for example, a reaction product solution produced by the esterification reaction is formed with a layer composed of a difluoroacetic acid ester and a tertiary amine/hydrogen fluoride salt and a layer composed of a free tertiary amine. These layers can easily be separated to recover the tertiary amine. [0080] The method of contact between the pyrolysis product and the reaction reagent is not limited, it is possible to use a publicly-known gas-liquid contact method that is the same as the method described in the Absorption Separation Method. In these contact methods, it is also possible to use the same apparatuses or different apparatuses by combining them in series. In the case of the bubbling method using the bubbling column or bubble-stirred tank (Both of them may simply be referred to as "tank".), there is preferable a multi-tank form in which a plurality of tanks arc connected in series. As an example, either a single tank or a plurality of tanks may be used. There is preferable a multi-tank form in which a plurality of tanks are connected in series. As an example, the case of an absorbing tank group formed of three tanks is explained. Firstly, piping is set in a manner that the pyrolysis product is introduced into the first tank and that a component mainly containing monofluoromethane is recovered through the second tank. When the reaction reagent or acid acceptor of the reaction absorption liquid has been consumed, piping is changed in a manner to switch the target of introducing the pyrolysis product to the second tank and recover a component mainly containing monofluoromethane through the third tank. When the reaction reagent or acid acceptor of the reaction absorption liquid of the second lank has been consumed, piping is changed in a manner that the third tank is made to be the target of introducing the pyrolysis product and that the first tank prepared by taking the content liquid out and charging the same with a newly-prepared reaction absorption liquid is made to be an exit of a component mainly containing monofluoromethane. Hereinafter, it can be conducted similarly. [0081] In the contact with the reaction absorption liquid, monofluoromethane does not react, thereby causing no lowering of yield by the reaction. Normally, CH4, C2H4, CHF3. C3H6. etc. are contained in a component (a low-boiling-point component) obtained by the treatment of the Reaction Separation Method and mainly containing monofluoromethane, but it can be purified by fractional distillation. [0082] Difluoroacetie acid, difluoroacetic acid salt, difluoroacetic acid ester, and difluoroacetic acid amide, which have been produced by the Reaction Separation Method, can respectively be separated and purified by using publicly-known methods. For example, it is possible to recover a difluoroacetic acid ester by washing the reaction absorption liquid of the esterification reaction with water and/or a basic aqueous solution and then conducting a distillation. In the case of using the tertiary amine as a basic substance, it is possible to recover a difluoroacetic acid ester by immediately conducting a distillation on the reaction absorption liquid. [0083] [Purification Method] A low-boiling-point component obtained by the above-mentioned various separation methods may contain a very small amount of an acid component. For example, a low-boiling-point component after the separation by the cooling liquefaction may be contaminated with difluoroacetic acid fluoride or hydrogen fluoride by entrainment or the like. The acid component contained in the low- boiling-point component can be removed by the contact with water and/or a basic aqueous solution to conduct washing and then conducting a drying treatment to produce monofluoromethane of high purity. As to the washing method, it is possible to arbitrarily apply various gas-liquid contact methods shown in the Absorption Separation Method, such as a bubbling method using a bubble column, a scrubber method using a packed column, etc. The basic aqueous solution is exemplified by KOH aqueous solution, NaOH aqueous solution, Ca(OH)2 aqueous solution, etc. KOH aqueous solution is preferable, with which saturated solubility of a fluoride salt formed upon the contact is high, and with which it is difficult to make a trouble such as clogging of the apparatus. After the washing, it is desirable to remove water by a dehydrator, such as soda lime, synthetic zeolite, silica gel, etc. Furthermore, soda lime, synthetic zeolite, and silica gel may have an effect of removing undesirable by-products, besides dehydration. As the synthetic zeolite, it is possible to use 3A type, 4A type, 5A type, 10X type. 13X type, etc. By subjecting monofluoromethane obtained by the pyrolysis to only the washing and the step of drying by zeolite, soda lime, etc., it is possible to easily have a purity of 99% or higher without conducting a fractional distillation purification. In the case of conducting the pyrolysis under the optimal condition, it is possible to have a purity of 99.9% or higher. This is a purity that is sufficient for the use as an etching gas or cleaning gas in semiconductor industries. [0084] Furthermore, the low-boiling-point component may contain very small amounts of CH4. C2H4, CHF3, C3H6, etc. It is, however, possible to conduct a further purification by a fractional distillation. The fractional distillation can be conducted by a publicly-known method using a rectifying column packed with various packing materials. The fractional distillation of monofluoromethane (boiling point: -78°(") may be conducted under atmospheric pressure, but a pressurized distillation is convenient since it becomes a low-temperature distillation. [0085] A monofluoromethane-containing component, in which impurities have been concentrated b\ the distillation, can be made to be harmless and scrapped by burning it with air or oxygen, or burning it while it is brought into contact with the catalyst used in the pyrolysis of the method of the present invention, particularly a catalyst having a phosphate as an active component, al around 500°C with oxygen, etc. and then washing the produced exhaust gas with a basic aqueous solution. [0086] Monofluoromethane as a product is prepared by cooling and solidifying a monofluoromethane produced by the method of the present invention with liquid nitrogen, etc., removing the air component by reducing pressure of the inside of the container with a vacuum pump, then restoring to a gas or liquid condition, and then a transfer to a cylinder tor storage. The obtained product is a monofluoromethane that is superior in both organic matter purity and inorganic matter purity and that contains no air. etc. [0087] [Uses] Monofluoromethane is useful as a so-called etching gas (an etching agent) for etching thin films and thick films, which are produced by using CVD method, sputtering method, so-gel method, vapor deposition method, etc. in the thin-film device production process centering on semiconductor industries, the optical device production process, the super steel material production process, etc. Furthermore, it is also useful as a so-called cleaning gas for removing thin films and granular materials deposited on the device, piping, etc. during the production of thin films, etc. in these processes. [0088] Monofluoromethane obtained by the present invention is substantially free from halogens other than fluorine, such as chlorine, bromine, etc., which form deep impurity levels in semiconductors. Therefore, it is suitable for semiconductor production apparatuses and semiconductor thin-film process uses. This is caused by that there is no possibility of a contamination with these halogen elements other than fluorine, since chlorine, bromine, and iodine are not contained in the reaction raw material and secondary materials. A monofluoromethane by the method of the present invention containing no halogens such as chlorine is preferable, also in respect of that halogen elements, such as chlorine, are pointed out to have an effect on the etching rate or the anisotropic etching in etching. [0089] Monofluoromethane of the present invention or an etching gas containing the same can preferably be applied to W, WSix, Ti, TiN, Ta20>, Mo, Re, Ge, Si3N4, Si, SiOo. etc. deposited on substrates of silicon wafer, metal plate, glass, single crystal, polycrystal, etc. [0090] In the case of using monofluoromethane as an etching gas, there is preferably used an etching method using plasma, such as RIE (reactive ion etching), ECR (electronic cyclotron resonance) plasma etching, microwave etching, high- frequency plasma etching, etc. The treatment condition is not particularly an issue. It is also possible to add various additives, depending on the type, the properties, the productivity, the fine precision, etc. of the target film. Of inert gases such as N2, He, Ar. Ne, and Kr, particularly Ar makes it possible to obtain a higher etching rate by a synergy effect with monofluoromethane. When it is hoped to increase the etching rate in order to increase productivity, it is possible to add an oxidative gas. Specifically, it is exemplified by 02, 03, C02, F2, NF3. Cl2, Br2,12, and XFn (X - CI, J, Br. l x 5mmL with atableting machine, and then baking at 700°C for five hours in a nitrogen gas flow to prepare an aluminum phosphate/cerium phosphate catalyst. [0101] [CATALYST PREPARATION EXAMPLE 3] An aluminum phosphate of Aldrich was subjected to tablet compression into pellets of 5mm x 5mmL, followed by baking at 700°C for five hours in a nitrogen gas flow to prepare an aluminum phosphate catalyst. [0102] [CATALYST PREPARATION EXAMPLE 41 A stainless steel (SUS316) reaction tube equipped with a heating mantle and having a length of 1.5m and an inner diameter of 55mm was charged with 2kg of y- alumina beads (Sumitomo Chemical Co., Ltd., KHS-46). The heating mantle temperature was regulated at 50°C. While nitrogen (lOOOcc/min) was allowed to flow, hydrogen fluoride (HF) vaporized by a vaporizer was allowed to flow at 4g/min. By this flow, generation of heat (heat of adsorption and heat of reaction) was found particularly at the inlet portion, and the heat generating region gradually moved toward the outlet. Upon this, when the heat spot having the highest temperature exceeded 300°C. the HF supply rate was lowered to lg/min to suppress a local generation of heat. After confirming that the temperature became the preset temperature, the HF supply rate was made to gradually return to 4g/min. After the heat generating region reached around the outlet, the preset temperature of the jacket was increased to 250°C hy 50°C, followed by repeating the fluorination of the above y-alumina. Then, the jacket preset temperature was set at 300°C, and the HF flow rate was gradually increased to 20g/min. In case that the temperature of the heat spot upon this exceeded 350°C, the HF flow rate was lowered to 1 g/min. Under the conditions of the jacket temperature of 300°C and the 1 IF flow rate of 20g/min, from the time when the heat spot had substantially ceased to be observed, a fluorination treatment was further continued for 24 hours under the same conditions. Then, while only nitrogen was allowed to flow, the power of the heater was turned off, followed by cooling, thereby obtaining a tluorinated alumina catalyst. [0103] [CATALYST PREPARATION EXAMPLE 51 Anhydrous aluminum fluoride (AIF3) of Aldrieh was subjected to a tablet compression into pellets of 5mm(|> x 5mmL, followed by baking at 700°C in a nitrogen gas flow! for 5 hours, to prepare an aluminum fluoride catalyst. [0104] [REFERENCE EXAMPLE 1, Sensitivity Measurement of FID Detector] Using samples of monofluoromethane and difluoroacetic acid fluoride, the sensitivity measurement of FID detector was conducted. Monofluoromethane (40kPa, 300torr) and difluoroacetic acid fluoride (40kPa, 300torr) were taken into a cylinder (300cc) (molar ratio = 1:1, total pressure: 80kPa, 600torr), followed by heating the cylinder at 25°C. From this, a sample of 0.2cc was taken into a gas syringe, followed by conducting a gas chromatograph analysis using an EPA METHOD 624-suited column to determine the areal ratio. Area of CH3F : Area of CHF2COF = 2.41 : 1.00 [0105] [EXAMPLE 1J An apparatus used in the experiment is shown in Pig. 3. There was used a stainless steel reaction tube 51 having a Sampling Port A 53 at the exit side, being equipped with an electric furnace 52 on the outside, and having an inner diameter of 37mm and a length of 500mm. To the exit of the reaction tube 51, there was connected a jacketed, high-boiling-point compound collector 55 having two of stainless steel. Liebig condenser tubes 54 (a refrigerant of-50°C was allowed to flow) charged with stainless steel Raschig rings. Furthermore, a gas washing bottle A56 (content: water, cooling with ice 59), a gas washing bottle B57 (content: 50% KOH aqueous solution, cooling with ice 59), and a gas washing bottle C58 (void trap, cooling with ice 59), and a drying tube 60 charged with soda lime and synthetic zeol ite 4 A (1:1) were connected together in this order i n series. The exit of the drying tube was provided with a Sampling Port B 61. [0106] The catalyst (230cc) prepared by Catalyst Preparation Example 1 was fed to the reaction tube 51. While nitrogen was allowed to flow at 15 cc/min, the electric furnace 52 was heated. When the temperature of the catalyst reached 50°C, hydrogen fluoride (HF) was introduced through the vaporizer at 0.6 g/min. While HF was allowed to flow, the temperature was slowly raised till 210°C. It was maintained for 15 hours. The flow of HF was stopped, and the nitrogen flow rate was increased to 200 cc/min. It was maintained for 2 hours. Immediately after introducing l-metho\\ -1,1,2,2-tetrafluoroethane (HFE-254pc) through the vaporizer at 0.5 g/min, the flow oi" nitrogen was stopped. When the reaction temperature turned to a steady state at 300°C, a gas sample taken at the Sampling Port A 53 of the exit of the reaction tube was analyzed with a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector, and a gas resulting from the cooling, washing and drying treatments was sampled at the Sampling Port B 61 and analyzed with a gas chromatograph (silicon-based PLOT column) of an FID column. The results are shown in Table 1 and Table 2. [0109] [EXAMPLES 2-21] Using catalysis prepared by Catalyst Preparation Examples 2-5, experiments were conducted in the same way as that of Example 1 under conditions described in Table 1. The obtained results are shown in Table 1 and Table 2. [0110] [EXAMPLE 22] The apparatus used in the experiment is shown in Fig. 4. The pyrolysis reaction was conducted under the same condition as thai of Example 1. The pyrolyzed gas flowing out of the reaction tube 71 was passed through a coil 75 immersed in an ethanol bath kept at -15°C. Then, it was cooled by a separation column 78 (-15°C) having a reflux condenser 79 in which the column top was kept at -78°C by a dry ice/acetone bath, thereby condensing a high-boiling-point component and collecting it in a jacketed high-boiling-point compound collector 76 (-15°C). An uncondensed low-boiling-point component was passed through an iced water trap 81, a potassium hydroxide aqueous solution trap 82, and a drying tube 83 packed with Synthetic Zeolite 4A. The samples were taken out of Sampling Port A 73. Sampling Port B 84. Sampling Port C 80, and Sampling Port D 77, which were shown in Fig. 4. and analyzed by a gas chromatograph using an EPA METHOD 624-suited column. As to Sampling Port B 84 and Sampling Port C 80, the analysis was conducted by a silicon-based PLOT column, too. It was confirmed that the analysis results substantially coincided with each other between these columns. The results are shown in "fable 3. [0111] [0112] [REFERENCE EXAMPLE 2] A stainless steel reaction tube equipped with an electric furnace on the outside and having an inner diameter of 23mm and a length of 500mm was charged with a granular activated carbon G2X (50cc) made by Japan EnviroChemicals, Ltd. and then heated by the electric furnace while nitrogen was allowed to flow at 15 cc/minute. When the temperature of the inside of the reaction tube reached 200°C, the raw material 1-methoxy-l, 1,2,2-tetrafluoroethane (I IFE-254pc) was introduced at a rale of 0.4 g/minute through a vaporizer. When the temperature of the inside of the reaction tube turned to a steady state at 250°C, the produced gas was analyzed. As a result, conversion was 0.9%, so that in effect the raw material was recovered. [0113] [REFERENCE EXAMPLE 3] The reaction was conducted in the same way as that of Reference Example 2, except in that the temperature of the inside of the reaction tube was adjusted to 300°C. As a result conversion was 2.6%. Although the temperature was raised to 300°C, similarly the raw material was recovered. [0114] [REFERENCE EXAMPLE 4] A stainless steel reaction tube equipped with an electric furnace on the outside and having an inner diameter of 23mm and a length of 500mm was charged with a granular activated carbon G2X (50cc) made by Japan EnviroChemicals, Ltd. and then heated by the electric furnace while nitrogen was allowed to flow at 15 cc/minute. When the temperature of the inside of the reaction tube reached 50°C, IIP (0.6 g/minute) was introduced through a vaporizer. While HF was kept to flow, the temperature was slowly raised until 300°C and maintained for 5hr. The flow of HF was stopped. The nitrogen flow rate was increased to 200 cc/minute and maintained for 2hr. Then, the nitrogen flow rate was changed to 15 cc/minute, and l-methoxy-l,l,2,2-tetrafluoroethane (HFE-254pc) was introduced at a rate of 0.4 g/minute through a vaporizer. When it turned to a steady state at a temperature of the inside of the reaction of 300°C, the produced gas was analyzed. As a result, conversion was 2.8%. so that in effect the raw material was recovered. No effect was found, even if conducting a HF treatment on an activated carbon having no phosphoric acid supported. [0115] [EXAMPLE 23] An apparatus shown in Fig. 4 was used. There was used a stainless steel reaction tube 71 having Sampling Port A 73 on the exil side, equipped with an electric furnace 72 on the outside, and having an inner diameter of 37mm and a length of 500mm. There were each connected by fluororesin or polyethylene pipes a polyethylene void trap 74 at the exit of the reaction tube 71, a coil 75 in a refrigerant bath maintained at -15°C, a separation column 78 (-15°C) having at the column top a reflux condenser 79 kept at -78°C by a dry ice/acetone bath, at the column bottom a jacketed high-boiling-point compound collector 76, and at the exit side Sampling Port B 84. an iced water trap 81, a basic aqueous solution trap 82 (50% KOH aqueous solution, cooled with ice), and a drying tube 83 packed with Synthetic Zeolite 4A at 1:1. The exit of the drying tube 83 was opened to an abatement apparatus. [0116] Prior to the start of the experiment, in the apparatus shown in Fig. 4, the connection of the reaction tube 71 and the void trap 74 was broken, and the piping was changed such that a direct exhaust from the exit of the reaction tube 71 into the abatement apparatus became possible. The reaction tube 71 was charged with 230cc of the catalyst obtained by Catalyst Preparation Example 4. Then, the temperature of the electric furnace 72 was raised while nitrogen was allowed to flow at 15 cc/minute. When the temperature of the catalyst reached 50°C, hydrogen fluoride (HF) was introduced al 1.0 g/min through a vaporizer. While HF was allowed to flow, the temperature was slowly raised until 350°C. When a local heat generation was found during the temperature rise, the supply rate was lowered to 0.1 g/min. After confirming the termination of the local heat generation, the HF supply rate was slowly increased gradually until 1.0 g/min. When it reached 350°C, it was maintained for 30 hr. Then, the flow of HF was stopped. The nitrogen flow rate was increased to 200 ec/min, and it was maintained for 2 hr. Then, the temperature of the electric furnace was lowered to 180°C. l-metho\y-l,l,2,2-tetrafluoroethane (HFE-254pc) was introduced at a rate of 0.2 g/min through a vaporizer. Immediately after that, the How of nitrogen was stopped. The setting of the electric furnace 72 was changed so that the reaction temperature became 150°C. After it became a steady state, it was restored to the apparatus configuration shown in Fig. 4 by connecting the exit of the reaction tube with the void trap 74. The outflow gas was passed through the void trap 74 and the coil 75. Then, a high-boiling-point component was condensed by the separation column 78 (-15°C) and collected by the jacketed high-boiling-point compound collector 76 (-15°C). A low-boiling- point component not condensed was passed through the iced water trap 81, the potassium hydroxide aqueous solution trap 82, and the drying tube 83. The sample taken at Sampling Port A 73 was analyzed by a gas chromatograph (EPA METHOD 624-suited column) of an FID detector and found to be 54.291% of CH3F, 22.126% of CHF2COF, 23.101% of CHF2CF2OMe (Me represents a methyl group. It is the same in the following.), and 0.482% of others. Furthermore, the sample collected at Sampling Port B 84 was analyzed by a gas chromatograph (a silicon-based PLOT column) of an FID detector and was found to be less than 0.001%) of CH4, 0.017% of C2H4, 0.009% of CHF;„ 99.961% of CH3F, 0.008% of C3H6, and 0.004% of others. The results are shown in Table 1 and Table 2. [0117] [EXAMPLE 24] The same experiment as that of Example 23 was conducted, except in that the reaction temperature was 175°C. The sample taken at Sampling Port A 73 was analyzed by a gas chromatograph (EPA METHOD 624-suited column) of an FID detector and found to be 69.544% of CH3F, 28.240% of CHF2COF, 1.351% of CHF2CF2OMe. and 0.685% of others. Furthermore, the sample collected at Sampling Port B 84 was analyzed by a gas chromatograph (a silicon-based PLOT column) of an FID detector and was found to be 0.024% of CH4, 0.121% of C2H4, 0.126% of CHF3, 99.455% of CH3F, 0.003% of C3H6. and 0.271% of others. The results are shown in Table 1 and Table 2. [0118] [EXAMPLE 25] A stainless steel reaction tube having an inner diameter of 23mm and a length of 400mm was packed with a granular (particle size: about 2.5-3.5 mm) anhydrous calcium chloride (63g, bulk: 120cc) made b\ .lunsei Chemical Co., Ltd. While nitrogen was allowed to flow at 50 cc/min, it was heated to 160°C. The organic matter (CIIF:COF: 94.181%, and CHF2CF2OMc: 4.569%) collected by the jacketed high-boiling-point compound collector 76 in Example 24 was allowed to flow at 0.3 g/min, and at the same time the nitrogen supply was stopped. A heat generation of 10°C to 20°C was found at around the inlet, and the heat spot moved toward the exit over time. When the organic matter was supplied by 77.9g, the exit gas was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector and found to be 1.105% of CHF2COF, 4.708% of CH3C1, 0.001% of CHF2CF2OMe, 93.769% of CHF2COCl, and 0.417% of others. [0119] After the above analysis, the raw material was changed to CHF2CF2OMe (99.9%). and the reaction temperature was changed to 330°C. The exit gas under steady state (30 hours later) was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector. As a result, the composition was found to be 2.406% of C2H4, 68.486% of CH3F, 20.460 of CHF2COF, 7.656% of CHF2CF2OMe. and 0.992% of others. The results are shown in Table 1. [0120] After that, supply of the raw material was stopped. While nitrogen (100 cc/min) was allowed to flow, heating of the electric furnace was stopped to slowly cooling it to room temperature. The contents of the reaction tube were found to have some stains, but almost no pulverization and no cohesion were found, resulting in a configuration similar to that prior to the reaction. The contents were ground by an agate mortar and subjected to a powder XRD measurement. As a result, a diffraction pattern of CaF2 was shown. [0121] [REFERENCE EXAMPLE 5] The same experiment as that of Example 23 was conducted, except in that, in place of HFE-254pc. CHF2COF (a major impurity: CHF2CF2OMe, 1.1%) having a purity of 98.2% obtained by distilling the organic mailer recovered in the jacketed high-boiling-point compound collector 76 in Example 23 was supplied. The sample taken at Sampling Port A 73 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector and found to be 83.988% of CHF3, no detection of CH3F, no detection of CHF2COF. no detection of CHF2CF2OMe, and 16.012% of others. The sample collected at Sampling Port B 84 was analyzed by a gas chromatograph (a silicon-based PLOT column) of an FID detector and found to be 0.368% of CH4, 0.238% of C:H4, 92.653% of CHF3, 0.569% of CH3F. 0.176% of C3H6, no detection of CPlF2CF2OMe, and 5.996% of others. [0122] [REFERENCE EXAMPLE 6] The same experiment as that of Example 23 was conducted, except in that the reaction temperature was adjusted to 330°C. The sample taken at Sampling Port A 73 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector and found to be 26.013% of CH3F. 9.215% of CHF2COF, no detection (below the detection limit (0.001%)) of CHF2CF2OMe, and 64.772% of others. Furthermore, the sample collected at Sampling Port B 84 was analyzed by a gas chromatograph (a silicon-based PLOT column) and found to be 9.876% of CH4, 19.854% of C2II4, 28.812% of CHF3, 26.187% of CH3F. 4.877% of C3H6, no detection of CHF2CF2OMe, and 10.397% of others. The results are shown in Table 1 and Table 2. [0123] [EXAMPLE 26] The apparatus used in the experiment is shown in Fig. 5. There was used a stainless steel reaction tube 91 having Sampling Port A 93 on the exit side, equipped with an electric furnace 92 on the outside, and having an inner diameter of 37mm and a length of 500mm. By fluororesin or polyethylene pipes, there were connected in this order stainless steel Absorption Tank A 95 and Absorption Tank B 96, and a 300cc void trap 97 made of a fluororesin, which were immersed at the exit of the reaction tube 91 in a cooling medium bath maintained at -30°C, and a stainless steel drying tube 98 packed with 300cc of soda lime and equipped with Sampling Port B 99 on the exit side. The exit of the drying tube 98 was opened to an abatement apparatus. Absorption Tank A 95 and Absorption Tank B 96 were each charged with 170g of toluene. [0124] The reaction lube 91 was charged with 230cc of the catalyst obtained by Catalyst Preparation I Example 4. Then, the temperature of the electric furnace 92 was raised while nitrogen was allowed to How at 15 cc/minute. When the temperature of the catalyst reached 50°C, hydrogen fluoride (HF) was introduced at 1.0 g/min through a vaporizer. While HF was allowed to flow, the temperature was slowly raised until 350°C. When a local heat generation was found during the temperature rise, the supply .rate was lowered to 0.1 g/min. After confirming the termination of the local heat generation, the HF supply rate was slowly increased gradually until 1.0 g/min. When it reached 350°C, it was maintained for 20 hr. Then, the flow of HF \\ as stopped. The nitrogen flow rate was increased to 200 cc/min, and it was maintained for 2 hr. Then, the temperature of the electric furnace was lowered to 190°C. l-methoxy-l,l,2,2-tetrafluoroethane (HFE-254pc) was introduced at a rate of 0.10 g/min through a vaporizer. Immediately after that, the flow of nitrogen was stopped. [0125] The reaction was continued. When HFE-254pc of 132g (lmol) in total was allowed to flow through the reaction tube 91, the gas taken at Sampling Port A 93 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector. The results are shown in Table 1. [0126] The gas at the exit of the drying tube was passed through a stainless steel cylinder cooled with liquid nitrogen, thereby obtaining 29g of a collected matter. The collected matter was vaporized and analyzed by a silicon-based PLOT column. The results are shown in Table 2. Furthermore, this gas was analyzed by a gas chromatograph (an Il'A METHOD 624-suited column) of an FID detector and found to be 87.02 areal % of monofluoromethane, 12.70 areal % of toluene, and 0.28 areal % of others. [0127] The contents of Absorption Tank A 95 and Absorption Tank B 96 were combined together (424g, out of this, toluene is 340g) and subjected to a distillation using a pressurized distillation column (the number of theoretical stages: 10) packed with Dixon packing. As a result, difluoroacetic acid fluoride (84g) of a purity of 99.2% was obtained. [0128] [EXAMPLE 27] The apparatus used in the experiment is shown in Fig. 6. There was provided a stainless steel reaction tube 101 having Sampling Port A 103 on the exit side, equipped with an electric furnace 102 on the outside, and having an inner diameter of 37mm and a length of 500mm. By fluororesin or polyethylene pipes, there were connected in this order stainless steel Absorption Tank A 105 and Absorption Tank B 106, which were immersed at the exit of the reaction tube 101 in a cooling medium bath maintained at -30°C, a washing lank (water trap) 107 charged with 200cc of water, and a stainless steel drying tube 108 packed with 300cc of soda lime and equipped with Sampling Port B 109 on the exit side. The exit of the drying tube 108 was opened to an abatement apparatus. Absorption Tank A 105 and Absorption Tank B 106 were each charged with 200cc of ethanol. [0129] The reaction lube 101 was charged with 230cc of the catalyst obtained by Catalyst Preparation Example 4. Then, the temperature of the electric furnace 102 was raised while nitrogen was allowed to flow at 15 cc/minute. When the temperature of the catalyst reached 50°C, hydrogen fluoride (HF) was introduced at 1.0 g/min through a vaporizer. While HF was allowed to flow, the temperature was slowly raised until 350°C. When a local heat generation was found during the temperature rise, the supply rate was lowered to 0.1 g/min. After confirming the termination of the local heat generation, the HF supply rate was slowly increased gradually until 1.0 g/min. When it reached 350°C, it was maintained for 20 hr. Then, the flow of HF was stopped. The nitrogen flow rate was increased to 200 cc/min, and it was maintained for 2 hr. Then, the temperature of the electric furnace was lowered to 190°C. l-methoxy-l,l,2,2-tetrafluoroethane (HFE-254pc) was introduced at a rate of 0.10 g/min through a vaporizer. Immediately after that, the flow of nitrogen was slopped. [0130] The reaction was continued. When HFE-254pc of 132g (lmol) in total was allowed to flow through the reaction tube 101, the gas taken at Sampling Port A 103 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector, and at the same time the gas taken at Sampling Port B 109 was analyzed by a gas chromatograph (a silicon-based PLOT column) of an FID detector. The results are shown in Table 1 and Table 2. Furthermore, the gas taken at Sampling Port B 109 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) of an FID detector and found to be 99.67 areal % of mono fluoro methane and 0.33 areal % of other components. [0131] [EXAMPLE 28] The same experiment as that of Example 27 was conducted, except in that Absorption Tank A 105 and Absorption Tank B 106 were charged with 20% KOH aqueous solution (200ce each), in place of ethanol, thai the cooling temperature was adjusted to -2°C, and that the washing tank 107 was replaced with a void trap. The analysis results are shown in Table 1 and Table 2. Furthermore, the gas taken at Sampling Port B 109 was analyzed by a gas chromatograph (an EPA METHOD 624-suited column) and found to be 99.84 areal % of monofluoromethane and 0.16 areal % of other components. [0132] [MONOFLUOROMETHANE USE EXAMPLE 1] Monofluoromethane after the purification and the drying, obtained by Example 20 and Example 23, was cooled by liquid nitrogen and collected in a stainless steel cylinder. A degassing operation composed of solidification of the collected matter by liquid nitrogen, decompression by a vacuum pump, and melting (room temperature) was repeated three times, thereby removing an air component. There is shown an example in which an interlayer dielectric film (SiO2) has been etched by using this gas in a contact hole processing. In Fig. 1 A, a sectional diagram of a sample prior to the etching is schematically shown. In Fig. 1B, a sectional diagram of the sample after the etching is schematically shown. On a monocrystalline silicon wafer 21, a SiO2 interlayer dielectric film 22 was formed. On the SiO2 film, a resist mask 23 provided with an opening portion as an etching mask was formed. In Fig. 1B, a reference number of 24 represents a portion of the loss of a shoulder. [0133] In Fig. 2, there is shown a schematic sectional view of a remote plasma apparatus used in the experiment. The gas (monofluoromethane) and Ar were respectively introduced at 50SCCM and 20SCCM front a first gas introducing port 4 and a second gas introducing port 5. An excitation was conducted in a sapphire tube 7 attached to an upper part of a reaction chamber 1 by using a high-frequency power source 3 (13.56MHz. 50W). The generated active species was supplied into the chamber by a gas flow to conduct an etching of the sample 12 fixed to a sample holder 11. The etching gases were each introduced through a mass flow controller (not shown in the drawings). The substrate (the sample holder 11) temperature was set at 25°C. the pressure at 2.67 Pa (0.02 torr), and the R1 power density at 2.2 W/cm2. The relative etching rate was determined as an areal ratio of the area of SiF3 (mass number: 85) obtained by analyzing the exhaust gas of the reaction chamber 1 by a mass spectrograph to the area of S1F3 being 1.000 when using mo no fluorom ethane of a commercial product (Monofluoromethane Use Example 2). With this, the relative etching rates of monofluoromethanes prepared by Example 20 and Example 23 were respectively 1.002 and 1.001. The results are shown in Table 4. [MONOFLUOROMETHANE USE EXAMPLE 2] An etching test was conducted under the same conditions as those of Monofluoromethane Use Example 1 by using a commercial, semiconductor-grade (fineness value in the test result slip of the product: 99.99%) monofluoromethane. The results are shown in Table 4. INDUSTRIAL APPLICABILITY [0135] Monofluoromethane obtained by the method of the present invention is useful as a semiconductor gas (dry etching agent, and cleaning agent). Difluoroacetic acid fluoride produced as a by-product and its derivatives are useful compounds, which arc used for catalysts of various reactions, pharmaceutical and agrochemical intermediates, intermediates of functional materials, etc. EXPLANATION OF SYMBOLS [0136] 1: a chamber. 2: earth, 3: a high-frequency power source, 4: a first gas introducing port. 5: a second gas introducing port, 6: a third gas introducing port, and 7: a sapphire tube. 8: an induction coil, 9: an electronic pressure gauge, 10: an exhaust gas line, 11: a sample holder, and 12: a sample. 21: a silicon wafer. 22: a SiO2 interlayer dielectric film, 23: a resist mask, and 24: a shoulder loss portion. 51: a reaction tube, 52: an electric furnace, 53: Sampling Port A, 54: a Liebig condenser tube; 55: a jacketed high-boiling-point compound collector, 56: a water trap, 57: a basic aqueous solution trap, 58: a void trap. 59: an iced bath, 60: a soda lime tube, and 61: Sampling Port B. 71: a reaction tube, 72: an electric furnace, 73: Sampling Port A, 74: a void trap; 75: a coil, 76: a jacketed high-boiling-point compound collector, 77: Sampling Port D, 78: a separation column, 79: a reflux condenser. 80: Sampling Port C, 81: an iced water trap, and 82: a basic aqueous solution. 83: a drying tube, and 84: Sampling Port B. 91: a reaction tube, 92: an electric furnace, 93: Sampling Port A, 94: a cooling medium bath. 95: Absorption Tank A, 96: Absorption Tank B, 97: a void trap. 98: a drying tube, and 99: Sampling Port B. 101: a reaction tube, 102: an electric furnace, 103: Sampling Port A, 104: a cooling medium bath. 105: Absorption Tank A, 106: Absorption Tank B, 107: a water trap, 108: a drying tube, and 109: Sampling Port B. WE CLAIM: 1. A method for producing monofluoromethane, comprising at least a pyrolysis step in which l-methoxy-1,1,2,2-tetrafluoroethane is pyrolyzed while it is brought into contact with a catalyst, and a step in which monofluoromethane is collected from a pyrolysis product. 2. The method for producing monofluoromethane. according to claim 1, wherein the step of collecting monofluoromethane is a step comprising a step of separating monofluoromethane by liquefying a part of the pyrolysis product. 3. The method for producing monofluoromethane. according to claim 2, wherein the liquefaction of a part of the pyrolysis product is conducted by cooling. 4. The method for producing monofluoromethane. according to claim 3, wherein the cooling temperature is from -80 to -5°C. 5. The method for producing monofluoromethane. according to claim 1, wherein the step of collecting monofluoromethane is a step comprising a step of absorbing difluoroacetic acid fluoride into a solvent that is inert against difluoroacetic acid fluoride. 6. The method for producing monofluoromethane. according to claim 5, wherein the solvent that is inert against difluoroacetic acid fluoride is a hydrocarbon compound. 7. The method for producing monofluoromethane. according to claim 1, wherein the step of collecting monofluoromethane is a step comprising a step of bringing into contact with a compound that is active to difluoroacetic acid fluoride. 8. The method for producing monofluoromethane, according to claim 7, wherein the compound that is active to difluoroacetic acid fluoride is water, an alcohol, primary amine, secondary amine, or β,α-unsaturated carboxylic acid ester. 9. The method for producing monofluoromethane, according to claim 7 or claim 8, wherein a solvent is made present in the step of bringing into contact with the compound that is active to difluoro acetic acid fluoride. 10. The method for producing monofluoromethane. according to any one of claims 7-9, wherein a basic substance is made present in the step of bringing into contact with the compound that is active to difluoroacetic acid fluoride. 11. The method for producing monofluoromethane, according to any one of claims 1-10, wherein the pyrolysis step is conducted such that a metal oxide, a partially fluorinated metal oxide, a metal fluoride, an untreated or fluorinated phosphoric acid, or an untreated or fluorinated phosphate is used as the catalyst and that the pyrolysis temperature is made to be from 100°C to 400°C. 12. The method for producing monofluoromethane. according to any one of claims 1-10, wherein the pyrolysis step is conducted such that alumina, a partially fluorinated alumina, or aluminum fluoride is used as the catalyst and that the pyrolysis temperature is made to be from 130°C to 260°C. 13. An etching agent or cleaning gas in a semiconductor device production step, which is characterized by comprising monofluoromethane produced by the method according to any one of claims 1-12. 14. An etching method or cleaning method in a semiconductor device production step, which is characterized by using monofluoromethane produced by the method according to any one of claims 1-12. 15. The method for producing monofluoromethane, according to any one of claims 1-6. comprising a step of obtaining difluoroacetic acid fluoride by separating the same, with the obtainment of monotluoromethane.