DESCRIPTION PROCESS FOR PRODUCTION OF POLY(LACTIC ACID)-TYPE RESIN, AND POLY(LACTIC ACID)-TYPE PREPOLYMER TECHNICAL FIELD [0001] The present invention relates to a method for efficiently producing a poly(lactic acid) resin having a high molecular weight and, in a preferred embodiment, having a high melting point as well as excellent thermal stability and hue. BACKGROUND ART [0002] In recent years, from the viewpoint of environmental protection, poly(lactic acid) resins have been paid attention, and polylactic acid resins are especially have been paid attention as plant-based carbon neutral materials. Polylactic acid resins have melting points of as high as about 170°C and can be processed by melt-molding. Further, since lactic acid, which is the monomer for those resins, can now be produced inexpensively by a fermentation method using a microorganism, polylactic acid resins are expected as bioplastics which can replace the petroleum-based commodity plastics, and gradually becoming common. [0003] Major methods for production of polylactic acid resins are the ring-opening polymerization method by polymerization of lactide, which is a lactic acid dimer, by ring-opening, and the direct polycondensation method by dehydration polycondensation using lactic acid. The direct polycondensation method is said to be capable of more inexpensively producing a polylactic acid resin compared to the ring-opening polymerization method since the step of synthesizing lactide is not necessary and lactic acid can be directly used as a polymerization raw material. PRIOR ART DOCUMENTS [Patent Documents] [0004] Patent Documents 1 to 10 describe the direct polycondensation method. Patent Documents 1 to 3 describe methods for producing polylactic acid by direct melt polymerization. Patent Document 4 discloses a production method by combination of melt polymerization and solid-phase polymerization. Patent Documents 5 to 8 also disclose production methods by combination of melt polymerization and solid-phase polymerization. Patent Documents 9 and 10 also discloses production methods by combination of melt polymerization and solid-phase polymerization. [Patent Documentl] JP 8-183 840 A (pp. 1 to 4) [Patent Document] JP 2000-297145 A (pp. 1 to 8) [Patent Document3] JP 2000-297143 A (pp. 1 to 14) [Patent Document4] JP 11-106499 A (pp. 1 to 6) [Patent Document5] JP 2000-302852 A (pp. 1 to 32) [Patent Document6] JP 2001-192444 A (pp. 1 to 24) [Patent Document7] JP 2001-64375 A (pp. 1 to 10) [Patent Document8] JP 2009-144132 A (pp. 1 to 29) [Patent Document9] JP 2000-273165 A (pp. 1 to 11) [Patent Documentl0] WO2009/H2196 (pp. 1 to 77) SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0005] In the techniques described in Patent Documents 1 to 3, there is a problem in that the obtained molecular weight is low and use of a solvent is necessary for obtaining a high-molecular-weight product. Further, in the method of Patent Document 3, there is the problem of coloring of the polymer. [0006] In the technique described in Patent Document 4, the molecular weight after solid-phase polymerization is insufficient even when the molecular weight before the solid-phase polymerization was high. [0007] In the techniques described in Patent Documents 5 to 8, since crystallization in water, crystallization for a long time, or crystallization with hot air containing moisture is carried out before solid-phase polymerization, acidic compounds increase in the polymer, so that the rate of solid-phase polymerization and the yield of the polymer decrease, which is problematic. [0008] In the technique described in Patent Document 9, crystallization before solid-phase polymerization is insufficient, and a sufficient rate of solid-phase polymerization cannot be obtained. In the technique described in Patent Document 10, contacting with air occurs for a long time during the process from melt polymerization to solid-phase polymerization because of pulverization and the like, leading to an increase in acidic substances and hence resulting in a decreased solid-phase polymerization efficiency. [0009] The present invention aims to provide a method for efficiently producing a poly(lactic acid) resin having a high molecular weight and, in a preferred embodiment, having a high melting point as well as excellent thermal stability and hue. MEANS FOR SOLVING THE PROBLEMS [0010] As a result of a study for solving the above problems, the present inventors discovered a method for efficiently producing a poly(lactic acid) resin having a high molecular weight and, in a preferred embodiment, having a high melting point as well as excellent thermal stability and hue, thereby reaching the present invention. [0011] That is, the above object of the present invention can be achieved by a method for producing a poly(lactic acid) resin, the method comprising the steps of: carrying out direct polycondensation using lactic acid as a main raw material to prepare a crystallized prepolymer having a weight average molecular weight of 5,000 to 25,000, an enthalpy of fusion AHm of 50 to 65 [J/g] and an acid value A [mol/ton] satisfying the Inequality (1) below; and subjecting the crystallized prepolymer to solid-phase polymerization. [0012] 450/(Mw/10,000-0.14) oxalate, bis(2,2,6,6-tetrarnethyl-4-piperidyl)-malonate, bis(2,2,6,6-tetrametriyl-4-piperidyl)-sebacate, bis(2,2,6,6-tetramethyl-4-piperidyl)-adipate, bis(2,2,6,6-tetramethyl-4-piperidyl)-terephthalate, l,2-bis(2,2,6,6-tetramethyl-4-piperidyIoxy)-ethane, a,a'-bis(2,2,6,6-tetramethyl-4-piperidyloxy)-p-xylene, bis(2,2,6,6-tetramethyl-4-piperidyJtolylene-2,4-dicarbamate, bis(2,2,6,6-tetramethyl-4-piperidyl)-hexamethylene-1,6-dicarbamate, tris(2,2,6,6-tetramethyl-4-piperidyl)-benzene-l,3,5-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-benzene-l,3,4-tricarboxylate, l-[2-{3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy}butyl]-4-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy]2,2,6,6-tetramethylpiperidine, condensates of 1,2,3,4-butanetetracarboxylic acid, l,2,2,6,6-pentamethyl-4-piperidinol and p,p,P',(3',-tetramethyl-3,9-[2,4,8,10-tetraoxaspiro(5,5)undecane]diethanol, and polycondensates of succinic acid dimethyl-l-(2-hydroxyethyl)-4-hydroxy-2,2,6,6,-tetraraethylpiperidine; and polyamines such as 3,9-bis[2-(3,5-diamino-2,4,6-triazaphenyl)ethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane, ethylenediamine-tetraacetic acid, alkali metal (Li, Na, K) salts of ethylenediamine-tetraacetic acid, N,N'-disalicylidene-ethylenediamine, N,N'-disalicylidene-l,2-propylenediamine, N,N"-disalicylidene-N'-methyl-dipropylenetriamine and 3-salicyloylamino-l,2,4-triazole. Among these, at least one of aromatic amine compounds, alkylamine compounds having not less than 4 carbon atoms, and amine compounds having a pyrimidine skeleton is preferably contained. [0111] Specific examples of the alkali metal compounds include lithium compounds such as lithium isopropoxide, lithium chloride, lithium acetate, lithium lactate, lithium octoate, lithium stearate, lithium naphthenate, lithium t-butyl carbonate, lithium sulfate and lithium oxide; sodium compounds such as sodium isopropoxide, sodium chloride, sodium acetate, sodium lactate, sodium octoate, sodium stearate, sodium naphthenate, sodium t-butyl carbonate, sodium sulfate and sodium oxide; and potassium compounds such as potassium isopropoxide, potassium chloride, potassium acetate, potassium lactate, potassium octoate, potassium stearate, potassium naphthenate, potassium t-butyl carbonate, potassium sulfate and potassium oxide. In particular, least one of organic carboxylic acid alkali metal compounds having not less than 4 carbon atoms is preferably contained. [0112] Examples of the alkaline earth metal compounds include magnesium compounds such as magnesium diisopropoxide, magnesium chloride, magnesium acetate, magnesium lactate, magnesium stearate, magnesium carbonate, magnesium sulfate and magnesium oxide; calcium compounds such as calcium diisopropoxide, calcium chloride, calcium acetate, calcium octoate, calcium naphthenate, calcium lactate, calcium stearate and calcium sulfate; barium compounds such as barium diisopropoxide, barium chloride, barium acetate, barium octoate, barium naphthenate, barium lactate, barium stearate and barium sulfate. In particular, at least one of organic carboxylic acid alkaline earth metal compounds having not less than 4 carbon atoms is preferably contained. [0113] The amount of the amine compound(s), alkali metal(s) and/or alkaline earth metal(s) to be added is not restricted, and, in view of achieving excellent hydrolysis resistance, the amount is preferably 0.001 to 2 parts by weight, more preferably 0.01 to 1 part by weight, still more preferably 0.05 to 0.5 part by weight, most preferably 0.08 to 0.3 part by weight with respect to 100 parts by weight of the poly(lactic acid) resin. In cases where an amine compound is used, the ratio of the molar amount of nitrogen atoms of the amine compound with respect to the molar amount of sulfur atoms of the sulfur-containing compound containing sulfur having an oxidation number of not less than +5 as a catalyst in the polymer after solid-phase polymerization is preferably 0.3 to 0.9, more preferably 0.4 to 0.8. The timing of addition of the amine compound(s), alkali metal(s) and/or alkaline earth metal(s) is not restricted, and may be either before the beginning or after the completion of either the melt polymerization step or the solid-phase polymerization step. In view of obtaining a poly(lactic acid) resin having a high melting point and a high molecular weight, the compound(s) is/are preferably added at the stage of the melt polymerization step, and, in view of achieving excellent productivity, the compound(s) is/are more preferably added immediately before the completion of the above-described melt polymerization conditions 1 (140°C to 160°C, 13.3 to 66.6 kPa) or at the beginning of the above-described melt polymerization conditions 2 (160°C to 180°C, 1.3 to 6.5 kPa), still more preferably added both immediately before the completion of the melt polymerization conditions 1 and at the beginning of the melt polymerization conditions 2. Further, similarly in view of achieving excellent productivity, the compound(s) is/are preferably added after addition of the sulfur-containing compound containing sulfur having an oxidation number of not less than +5 as a catalyst. In cases where the compound(s) is/are added at the beginning of the melt polymerization conditions 2, the catalyst for solid-phase polymerization is preferably added after the addition of the amine compound(s), alkali metal(s) and/or alkaline earth metal(s). In cases where the compound(s) is/are added at the stages of both the melt polymerization conditions 1 and the melt polymerization conditions 2, the compound(s) is/are preferably added in an amount of 0.001 to 1 part by weight at each stage, and, in view of achieving excellent productivity, die compound(s) is/are more preferably added in an amount of 0.01 to 0.5 part by weight at each stage, still more preferably added in an amount of 0.01 to 0.1 part by weight at each stage with respect to 100 parts by weight of the poly(lactic acid) resin. In view of obtaining a poly(lactic acid) resin having excellent hydrolysis resistance, the compound(s) may also be preferably added after completion of the solid-phase polymerization step. The method of addition of the amine compound(s), alkali metal(s) and/or alkaline earth metal(s) is not restricted, and examples of the method include a method wherein melt kneading is carried out at a temperature higher than the melting point of the poly(lactic acid) resin and a method wherein the compound(s) is/are dissolved in a solvent and the resulting solution is mixed, followed by removal of the solvent. In view of efficient production, the method wherein melt kneading is carried out at a temperature higher than the melting point of the poly(lactic acid) resin is preferred. The method of melt kneading may be either a batch method or continuous method, and examples of the apparatus which may be used include single screw extruders, twin screw extruders, multi-screw extruders, plastomill, kneaders and stirred tank reactors equipped with a pressure reducing device. In view of efficient uniform kneading, a single screw extruder or twin screw extruder is preferably used. The temperature at which the amine compound(s), alkali metal(s) and/or alkaline earth metal(s) is/are added is preferably a temperature of 180 to 250°C, and, in view of achieving excellent mechanical properties, a temperature of 190 to 230°C is more preferred. The pressure at which the amine compound(s), alkali metal(s) and/or alkaline earth metal(s) is/are added may be any of a reduced pressure, normal pressure and increased pressure. In view of removal of gas generated during melt kneading, the pressure is preferably a reduced pressure. In terms of the atmospheric conditions during the melt kneading, the melt kneading may be carried out either in the air or under an atmosphere of an inert gas such as nitrogen. In view of reduction in the amount of gas generated during the melt kneading, the melt kneading is preferably carried out under an atmosphere of an inert gas. [0114] In cases where the mixing is carried out in a solvent, a solvent that dissolves the polymer and monomers is used. Examples of the solvent which may be used include chloroform, methylene chloride and acetonitrile. In cases where the solvent needs to be removed after the mixing, the method for removing the solvent is not restricted, and examples of the method which may be used include a method wherein the solvent is evaporated at room temperature and a method wherein the solvent is evaporated under reduced pressure at a temperature higher than the boiling point of the solvent. [0115] The crystallized poly(lactic acid) prepolymer prepared in the present invention has a weight average molecular weight of 5,000 to 25,000, and the weight average molecular weight is preferably 10,000 to 20,000. The enthalpy of fusion AHm is 50 to 65 [J/g], more preferably 53 to 60. [0116] The acid value A [mol/ton] of the crystallized prepolymer needs to satisfy the Inequality (1) below, preferably satisfies the Inequality (5) below, more preferably satisfies the Inequality (6) below. [0117] 450/(Mw/10,000-0.14) (resorcinol, salicylate, benzotriazole, benzophenone and the like), lubricants, releasing agents (montanic acid and salts thereof, esters thereof and half esters thereof, stearyl alcohol, stearamide, polyethylene wax and the like), coloring agents including dyes (nigrosine and the like) and pigments (cadmium sulfide, phthalocyanine and the like), anti-coloring agents (phosphites, hypophosphites and the like), flame retardants (red phosphorus, phosphoric acid esters, brominated polystyrene, brominated polyphenylene ether, brominated polycarbonate, magnesium hydroxide, melamine, cyanuric acid and salts thereof, and the like), electrically conducting agents or coloring agents (carbon black and the like), tribological property improving agents (graphite, fluorine resins and the like), nuclear agents (inorganic nucleating agents including talc; organic amide compounds including ethylenebislauric acid amide, ethylenebis-12-dihydroxystearic acid amide and trimesic acid tricyclohexylamide; pigment nucleating agents including copper phthalocyanine and Pigment Yellow 110; organic carboxylic acid metal salts; phenylphosphonic acid zinc; and the like) and antistatic agents. [0122] The poly(lactic acid) resin composition obtained by the production method of the present invention may additionally contain at least one of other thermoplastic resins (polyethylene, polypropylene, acrylic resins, polyamide, polyphenylene sulfide resins, polyether ether ketone resins, polyester, polysulfone, polyphenylene oxide, polyacetal, polyimide, polyetherimide and the like), thermosetting resins (phenol resins, melamine resins, polyester resins, silicone resins, epoxy resins and the like), soft thermoplastic resins (ethylene/glycidyl methacrylate copolymers, polyester elastomers, polyamide elastomers, ethylene/propylene terpolymers, ethylene/butene-1 copolymers and the like) and the like as long as the object of the present invention is not adversely affected. [0123] The poly(lactic acid) resin composition obtained by the production method of the present invention, even after being once heat-melted and solidified upon processing into a molded article or the like, has a high molecular weight, and is likely to form a poly(lactic acid) resin having high heat resistance as well as excellent thermal stability and hue in a preferred embodiment. EXAMPLES [0124] The present invention will now be described more specifically by way of Examples. The number of parts in Examples herein represents parts by weight. [0125] The measurement methods and judgment methods used in the present invention were as follows. (1) Weight Average Molecular Weight This is the value of weight average molecular weight in terms of a poly(methyl methacrylate) standard as measured by gel permeation chromatography (GPC) using hexafluoroisopropanol as a solvent. (2) Acid Value The polymer was dissolved in an o-cresol/chloroform-mixed solvent (volume ratio, 2:1), and the resulting solution was subjected to neutralization titration with a potassium hydroxide/ethanol solution at a known concentration to measure the acid value. (3) Enthalpy of Fusion before Solid-phase Polymerization Under nitrogen atmosphere, the temperature was increased from 30°C to 200°C at a rate of 20°C/min. to measure the enthalpy of fusion by differential scanning calorimetry (DSC). (4) Melting Point after Solid-phase Polymerization Under nitrogen atmosphere, the temperature was kept at 200°C for 2 minutes and then decreased to 30°C at a rate of 20°C/min., followed by increasing the temperature at a rate of 20°C/min. to 200°C to measure the melting point by differential scanning calorimetry (DSC). (5) Amount of D-isomer The polymer was hydrolyzed with a sodium hydroxide solution and neutralized with hydrochloric acid. The amount of the D-isomer was then measured with a liquid chromatography to which an optical resolution column was attached. (6) Lactide Content The polymer was dissolved in deuterated chloroform and subjected to measurement by proton NMR. The lactide content was calculated based on the ratio between the areas of the peaks derived from lactide and the polymer. (7) Hue The hue was evaluated according to the following standards based on visual observation: 5: Colorless; 4: Intermediate between 3 and 5; 3: Colored in pale yellow; 2: Intermediate between 1 and 3; 1: Colored in yellow. [Example 1] In a reaction vessel equipped with an agitator and a reflux condenser, 100 parts by weight of 90 wt% aqueous L-lactic acid (amount of the D-isomer, 0.4%; manufactured by Wako Pure Chemical Industries, Ltd.) solution was placed, and, as catalysts, tin(II) acetate (manufactured by Kanto Chemical Co., Inc.) was added at 120 ppm in terms of tin atoms with respect to L-lactic acid (excluding water contained together in the material) and methanesulfonic acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added at 1,100 ppm in terms of sulfur atoms with respect to L-lactic acid (excluding water contained together in the material). The temperature was adjusted to 155°C and the pressure was gradually decreased to 700 Pa. The reaction was allowed to proceed for 3.5 hours while water was removed, and the polymerization reaction was then carried out at a temperature of 175°C at a pressure of 400 Pa for 7 hours, to obtain a prepolymer having a weight average molecular weight of 14,700. The obtained prepolymer was dropped onto a belt under nitrogen atmosphere without allowing contact with the ambient air, and cooled, to obtain a pellet in the shape of a ball having a diameter of 3 mm. Immediately thereafter, the pellet was treated under nitrogen atmosphere at 80°C for 1 hour, at 100°C for 1 hour and then at 120°C for 1 hour, to allow crystallization. The crystallized prepolymer had an acid value of 452 [mol/ton] and an enthalpy of fusion of 55.3 [J/g]. Immediately thereafter, at a pressure of 50 Pa, the temperature was continuously increased from 140°C to 160°C for 20 hours (temperature increasing rate, 1 °C per hour), and solid-phase polymerization was then carried out at 160°C for 20 hours. In 100 parts by weight of the obtained poly(lactic acid) resin, 0.2 part by weight of stearyl acid phosphate was mixed, and the resulting mixture was subjected to melt kneading using a biaxial extruder at 190°C. Properties of the obtained poly(lactic acid) resin were as shown in Table 1. [Examples 2 to 15, Comparative Examples 1 to 8] The operation was carried out in the same manner as in Example 1 except that the conditions for melt polymerization, conditions for crystallization of the prepolymer and conditions for solid-phase polymerization were as shown in Tables 1 and 2. The results are shown in Tables 1 and 2. It should be noted that, in Example 9, the length of time of temperature increase in the solid-phase polymerization was increased to 25 hours. In Comparative Examples 2 to 8, the production of the pellet was carried out in the air. [0126] [Table 1] [0127] ---[Table 2] [0128] [Table 3] INDUSTRIAL APPLICABILITY [0129] The poly(lactic acid) resin composition obtained by the production method of the present invention can be widely used as a molded article. Examples of the molded article include films, sheets, fibers/cloths, non-woven fabrics, injection-molded articles, extrusion-molded articles, vacuum/pressure-molded articles, blow-molded articles and complexes with other materials. These molded articles are useful for agricultural materials, garden materials, fishery materials, civil engineering and construction materials, stationery, medical supplies, automobile parts, electrical/electronic components and other uses. CLAIMS 1. A method for producing a poIy(iactic acid) resin, said method comprising the steps of: carrying out direct polycondensation using lactic acid as a main raw material to prepare a crystallized prepolymer having a weight average molecular weight of 5,000 to 25,000, an enthalpy of fusion AHm of 50 to 65 [J/g] and an acid value A [mol/ton] satisfying the Inequality (1) below: 450/(Mw/l 0,000-0.14)