DESCRIPTION TITLE OF THE INVENTION: METHOD FOR IMPROVING INHIBITION PERFORMANCE OF SEMIPERMEABLE MEMBRANE, SEMIPERMEABLE MEMBRANE, AND SEMIPERMEABLE MEMBRANE WATER PRODUCTION DEVICE TECHNICAL FIELD [0001] The present invention relates to enhancing of the performance of a semipermeable membrane used for obtaining a low-concentration permeate by using raw water such as seawater, saline river water, groundwater, lake water and treated wastewater. More specifically, the present invention relates to a rejection performance-enhancing method capable of enhancing the rejection performance of a semipermeable membrane. BACKGROUND ART [0002] In recent years, depletion of water resources has become serious, and use of water resources which have not been heretofore utilized is being studied. In particular, a technique for producing potable water from seawater which is most familiar and cannot be utilized as it is, so-called "seawater desalination", and furthermore, a recycling technique of purifying sewage or wastewater and desalinating the treated water, are attracting attention. The seawater desalination has been conventionally put to practical use mainly by an evaporation method in the Middle East area where water resources are extremely scarce and thermal resources from oil are very abundant, but in the regions other than the Middle East, where thermal resources are not abundant, an energy-efficient reverse osmosis method has been employed, and with the recent enhancement of reliability and reduction in cost owing to technical progress in the reverse osmosis method, a seawater desalination plant utilizing a reverse osmosis method is being constructed in many regions including the Middle East and showing global expansion. [0003] Recycling of sewage or wastewater is starting to be applied to inland or coastal cities or industrial districts, in which there is no fresh water source or the outflow rate is limited by effluent regulations. Among others, in Singapore that is an island country lacking water sources, water shortage is resolved by treating sewage generated in the country, storing the treated water without discharging it into sea, and reclaiming water at a potable level by means of a reverse osmosis membrane. [0004] The reverse osmosis method applied to seawater desalination or recycling of sewage or wastewater can produce desalinated water by passing water containing a solute, such as salt, through a semipermeable membrane at a pressure not less than the osmotic pressure. This technique also makes it possible to obtain potable water from, for example, seawater, brine or harmful substance-containing water and has been used as well, e.g., for the production of industrial ultrapure water, for the wastewater treatment, or for the recovery of a valuable substance. [0005] In order to stably operate a desalination apparatus using a reverse osmosis membrane, a pretreatment according to the quality of raw water taken is necessary. If the pretreatment is insufficient, the reverse osmosis membrane may be deteriorated or fouling (membrane surface fouling) may occur, and stable operation tends to become difficult. In particular, when a chemical substance deteriorating the reverse osmosis membrane enters the reverse osmosis membrane, an irreversible fatal situation may be caused. More specifically, the functional layer (the portion exerting a reverse osmosis function) of the reverse osmosis membrane is decomposed, and the performance of separating water from a solute, i.e., the solute rejection performance, is degraded. In the case of using a reverse osmosis membrane for applications such as seawater desalination or recycling of sewage or wastewater, it is very difficult to 100% prevent occurrence of the decomposition of the functional layer of the reverse osmosis membrane, and among others, polyamide that is the mainstream of the reverse osmosis membrane is susceptible to oxidative deterioration (Non-Patent Document 1). [0006] In addition, despite having some degree of durability, decomposition of the functional layer is likely to occur as well when exposed to a strong acid or alkali. Once such decomposition occurs, in the case of a semipermeable membrane having an anionic charge, which is a general reverse osmosis membrane for water treatment, the charge elimination effect of the anionic charge may give a greater adverse influence on removal of neutral molecules than on separation and removal of a rejectable inorganic electrolyte, and the rejection ratio of, among others, neutral molecules is reduced. Specifically, silica, boron, sugars, etc., which are not dissociated in a neutral region, causes a prominent decline in the water quality. Usually, the reverse osmosis membrane having lost the required rejection performance must be replaced with a new one, naturally leading to an increase in the treatment cost. [0007] Accordingly, development of a technique for recovering the rejection performance of a reverse osmosis membrane is proceeding for many years, and there have been proposed a number of methods for recovering the rejection performance of a reverse osmosis membrane and a number of recovering agents therefor, such as a method of contacting and reacting a vinyl-based polymer (Patent Documents 1 and 2), a method of contacting a polyethylene glycol with the reverse osmosis membrane to enhance the rejection ratio, particularly, the rejection ratio for a nonionic solute (Patent Documents 3 and 4), a method of contacting a nonionic surfactant with the membrane surface of a reverse osmosis membrane having an anionic charge and being increased in the permeation flux (Patent Document 5), a method of contacting an iodine and/or iodine compound having an oxidation-reduction potential of 300 mV or more (Patent Document 6), and a method of contacting an aqueous solution of a strong mineral acid such as phosphoric acid, phosphorous acid and sulfuric acid, raising the temperature and then contacting a rejection performance enhancer such as hydrolyzable tannic acid (Patent Document 7). These treatments for recovering the rejection performance have various technical issues. [0008] That is, depending on the kind or state (fouling, deterioration) of the reverse osmosis membrane, the treatment environment such as water temperature, or the conditions at the time of conducting the treatment (e.g., temperature of treatment liquid, concentration, treatment time), the effect of the rejection performance-enhancing treatment may vary, or reduction in the water permeation performance, which is in a sense a side effect of the rejection performance-enhancing treatment, may also vary. In addition, for example, the performance long-sustaining effect after enhancing the rejection ratio varies as well, and difficulty is often involved, for example, the water quality at the time of fresh-water generation operation after the rejection performance-enhancing treatment may be insufficient, or the operation pressure may be inadequate. [0009] Because, in large plants for seawater desalination or sewage recycling, which have been rapidly constructed and started running since entering the 2000s, a large number of reverse osmosis membranes are used or raw water in natural environments, such as seawater, is treated and therefore, even if a pretreatment is performed, the reverse osmosis membrane is operated while being subject to influence from season, rise and fall of tide, red tide, and other weather or natural environments, as a result, the reverse osmosis membrane assumes a variety of states in the same plant. In addition, for implementing the rejection performance-enhancing treatment, the normal fresh¬water generation treatment is once stopped, and the raw water to be treated at the time of operation is then replaced by a rejection performance enhancer through a chemical cleaning line, which is attended by many problems, for example, the utilization rate is reduced, complicated efforts are required, or unless the rejection performance or water permeation performance is also measured under normal operation conditions by again passing raw water to be treated after the completion of the treatment, the final effect is not judged. [0010] With respect to these problems, in order to solve the influence on the treatment effect due to difference in the state of reverse osmosis membrane, for example, as illustrated in Patent Document 8, a technique of cleaning the reverse osmosis membrane with chemicals and thereafter applying a rejection performance-enhancing treatment is adopted in genera]. Furthermore, as described in Patent Document 9, a pretreatment of cleaning the membrane with high-temperature water and contacting a rejection performance enhancer has also been proposed. As to the method forjudging the effect of the rejection performance-enhancing treatment, a method of confirming the treatment effect by adding a substance becoming a marker to a rejection performance enhancer and delecting the concentration of the marker substance in the permeate has been proposed (Patent Document 10). A method of determining the completion of treatment by monitoring the feed concentration and discharge concentration of rejection performance enhancer so as not to spend wasted recovering treatment lime any more after saturation of the rejection ratio-enhancing treatment is reached, has also been proposed (Patent Document 11). [0011] However, in these methods, only a relative treatment effect is known by seeing the performance during the rejection performance-enhancing treatment, the performance in an actual operation environment is difficult to grasp, and since the efficiency or effect of the treatment can be hardly controlled in the first place, it is often a practice to rely on on-the-spot try and error. BACKGROUND ART DOCUMENT PATENT DOCUMENT [0012] Patent Document 1: JP-A-55-114306 Patent Document 2: JP-A-59-30123 Patent Document 3: JP-A-2007-289922 Patent Document 4: JP-A-2008-132421 Patent Document 5: JP-A-2008-86945 Patent Document 6: JP-A-201 1-161435 Patent Document 7: JP-A-2-68102 Patent Document 8: JP-A-2008-36522 Patent Document 9: JP-A-2009-22888 Patent Document 10: JP-A-2008-155123 Patent Document 1 1: JP-A-2008-] 83488 NON-PATENT DOCUMENT [0013] Non-Patent Document 1: Tadahiro UEMURA, et al., "Chlorine Resistance of Composite Reverse Osmosis Membranes and Changes in Membrane Structure and Membrane Separation Properties Caused by Chlorination Degradation", Bulletin of the Society of Sea Water Science, Japan, Vol. 57, No. 3 (2003) Non-Patent Document 2: M. Taniguchi, et al., "Boron Reduction performance of reverse osmosis seawater desalination process", Journal of Membrane Science, 183, 259-267 (2000) SUMMARY OF THE INVENTION PROBLEMS THAT THE INVENTION IS TO SOLVE [0014] An object of the present invention is to provide a rejection performance-enhancing method enabling a semipermeable membrane such as nanofiltration membrane or reverse osmosis membrane to enhance the rejection performance of the semipermeable membrane, particularly, the rejection performance for nonionic substances, a semipermeable membrane and a semipermeable membrane element each treated by the rejection ratio-enhancing method, and a fresh-water generation apparatus and a fresh-water generation method each using a semipermeable membrane having enhanced rejection performance. MEANS FOR SOLVING THE PROBLEMS [0015] In order to solve the above-described problem, the present invention has the following configurations. (1) A semipermeable membrane rejection performance-enhancing method which is a method of enhancing a rejection performance of a semipermeable membrane by pressurizing and feeding a liquid containing a rejection performance enhancer to a primary side of the semipermeable membrane to come into contact with a membrane surface thereof, the method including: a step of changing, at least once during the feeding, a pressure or an osmotic pressure of the liquid containing the rejection performance enhancer at a fluctuation rate of 0.05 MPa/s or more, or a feed flow rate to the semipermeable membrane, thereby changing at least either a pressure acting on the membrane surface or a permeation flow rate from that at the time of normal treatment, followed by maintaining. (2) The semipermeable membrane rejection performance-enhancing method according to (1), in which a time for which the pressure or the permeation flow rate is maintained is from 10 seconds to 10 minutes. (3) The semipermeable membrane rejection performance-enhancing method according to (1) or (2), in which the permeation flow rate is fluctuated to 0.8 times or less or 1.2 times or more the permeation flow rate at the time of normal treatment, at least once during the feeding. (4) The semipermeable membrane rejection performance-enhancing method according to any one of (1) to (3), in which the change of the permeation flow rate is caused by a pressure change at least on the primary side or a secondary side of the semipermeable membrane. (5) The semipermeable membrane rejection performance-enhancing method according to any one of (1) to (4), in which the permeation flow rate is set to 0.1 times or less the permeation flow rale at the time of normal treatment. (6) The semipermeable membrane rejection performance-enhancing method according to any one of (1) to (5), in which a feed direction to the semipermeable membrane is reversed at least once. (7) The semipermeable membrane rejection performance-enhancing method according to any one of (1) to (6), in which the liquid containing the rejection performance enhancer contains a solute different from the rejection performance enhancer, and when a constant X is determined according to the kind of the rejection performance enhancer and a quantity of the treatment liquid fed to the semipermeable membrane is denoted as QFT [m3/day], a quantity of the treatment liquid permeated through the semipermeable membrane is denoted as QPT [m3/day], a membrane area of the semipermeable membrane is denoted as A [m ], a rejection performance enhancer concentration is denoted as C [mg/ij, a liquid transit time is denoted as t [h], and an osmotic pressure of the fed liquid is denoted as n, the treatment is applied to satisfy: 1.0X as an initial water permeation performance and a solute permeation coefficient Bo as a rejection performance are calculated from the measured values; while feeding and passing the rejection performance-enhancing treatment liquid to the reverse osmosis membrane, at least two fluids out of feed water, permeate and concentrate are measured for a flow rate, concentration and water temperature at that time; a pure water permeation coefficient At as an initial water permeation performance and a solute permeation coefficient B2 as a rejection performance are calculated from the measured values; in a case where Bj/Bo is not more than a predetermined value RQ when Aj/Ao becomes RAi or less, the rejection performance-enhancing treatment is terminated; in a case where Bi/Bo exceeds RB, the rejection performance-enhancing treatment is continued; and at a point where B|/Bo becomes RB or less or Aj/Ao is reduced to RA2, the treatment is stopped. (24) The semipermeable membrane rejection performance-enhancing method according to (23), in which RAi is 0.9 or less, RA2 is 0.7 or more, and RB is from 0.3 to 0.7. (25) The semipermeable membrane rejection performance-enhancing method according to (23) or (24), in which the pure water permeation coefficient A is a value corrected to a value at the lowest temperature TL at the time of operating the semipermeable membrane and the solute permeation coefficient B is a value corrected to a value at the highest temperature TH at the time of operating the semipermeable membrane, or both A and B are values corrected to the same temperature. (26) The semipermeable membrane rejection performance-enhancing method according to any one of (1) to (25), in which a liquid containing a rejection performance enhancer offering a rejection ratio of 99.9% or more in the semipermeable membrane is added to pretreated water obtained by pretreating raw water and thereafter, while producing permeate by separation treatment with the semipermeable membrane, added to feed water to the semipermeable membrane. (27) A semipermeable membrane or a semipermeable membrane element having rejection performance enhanced by the semipermeable membrane rejection performance-enhancing method according to any one of (1) to (26). (28) The semipermeable membrane or the semipermeable membrane element according to (27), which includes polyamide. (29) A semipermeable membrane fresh-water generation apparatus loaded with the semipermeable membrane or the semipermeable membrane element according to (27) or (28). ADVANTAGE OF THE INVENTION [0016] According to the rejection ratio-enhancing method of the present invention, when the permeate quality is degraded due to reduction in the rejection performance of a nanofiltration membrane or a reverse osmosis membrane in a fresh-water generation apparatus such as seawater desalination or sewage recycling, the rejection performance can be improved while minimizing the reduction in water permeation performance of a semipermeable membrane, and the water quality of a removal target substance such as inorganic electrolyte or neutral molecular can thereby be efficiently improved. BRIEF DESCRIPTION OF THE DRAWINGS [0017] [Fig. 1] Fig. 1 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which the semipermeable membrane rejection performance-enhancing method according to the present invention can be applied. [Fig. 2] Fig. 2 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which the semipermeable membrane rejection performance-enhancing method according to the present invention can be applied by reversing the flow on the semipermeable membrane. [Fig. 3] Fig. 3 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which the semipermeable membrane rejection performance-enhancing method according to the present invention can be applied by switching to a reverse flow on the semipermeable membrane. [Fig. 4] Fig. 4 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which a rejection performance-enhancing method can be applied by arranging a plurality of semipermeable membranes according to the present invention in series and performing intermediate dilution. [Fig. 5] Fig. 5 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which a rejection performance-enhancing method can be applied by arranging a plurality of semipermeable membranes according to the present invention in series and performing intermediate dilution with a permeate from another system. [Fig. 6] Fig. 6 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus to which a rejection performance-enhancing method can be applied by arranging a plurality of semipermeable membranes according to the present invention in series and performing intermediate dilution with a concentrate from another system. [Fig. 7] Fig. 7 is a diagram illustrating one example of the process flow of a semipermeable membrane fresh-water generation apparatus in which a fresh-water generation operation is performed while applying the semipermeable membrane rejection performance-enhancing method according to the present invention. [Fig. 8] Fig. 8 is a diagram illustrating another example of the process flow of a semipermeable membrane fresh-water generation apparatus to which a rejection performance-enhancing method can be applied by arranging a plurality of semipermeable membranes according to the present invention in series and using a second semipermeable membrane concentrate. [Fig. 9] Fig. 9 is a diagram illustrating another example of the process flow of a semipermeable membrane fresh-water generation apparatus to which a rejection performance-enhancing method can be applied by arranging a plurality of semipermeable membranes according to the present invention in series and using a first semipermeable membrane permeate. [Fig. 10] Fig. 10 is a diagram illustrating one example of the process flow of a testing device used for measuring the effect produced by the rejection performance-enhancing method of Example. MODE FOR CARRYING OUT THE INVENTION [0018] Preferred embodiments of the present invention are described beiow by referring to the drawings. However, the scope of the present invention is not limited thereto. [0019] Fig. I illustrates one example of a semipermeable membrane separation apparatus to which the semipermeable membrane rejection performance-enhancing method of the present invention can be applied. In the case where the semipermeable membrane fresh-water generation apparatus illustrated in Fig. I is operated to generate fresh water, raw water fed through a raw water line 3 is temporarily stored in a raw water tank 2, then delivered to a pretreatment unit 4 by a raw-water feed pump 3, and pretreated. The pretreated water passes through an intermediate water tank 5, a feed pump 6 and a safety filter 7 and after boosting the pressure by a booster pump 8, is separated into a permeate and a concentrate in a semipermeable membrane unit 9 including a semipermeable membrane module. The permeate is stored in a product water tank 12 through a product water line 10a. The concentrate is discharged outside the system through a concentrate discharge line 1 la after recovering its pressure energy by an energy recovery unit 13. [0020] During the fresh-water generation operation, a feed water valve 16a, a permeate valve 17a and a concentrate valve 18a are opened, and a feed chemical valve 16b, a permeated chemical valve 17b and a concentrated chemical valve 18b are closed. [0021] A chemical circulation fine used at the time of applying the present invention includes a chemical tank 15, a chemical feed pump 19 and a chemical dosing unit 20 (20a, 20b), and while a chemical fed to the semipermeable unit 9 through the chemical feed line 14 and permeated (depending on the kind of the chemical, all solutes are rejected and in this case, only a solvent) is refluxed to the chemical tank 15 through a permeate line 10 and a permeated chemical line, a concentrated chemical that is not permeated is refluxed to the chemical tank 15 through a concentrate line 11 and a concentrated chemical line 1 lb. [0022] During the semipermeable membrane rejection performance-enhancing treatment, the feed water valve 16a, the permeate valve 17a and the concentrate valve 18a are closed, and the feed chemical valve 16b, the permeated chemical valve 17b and the concentrated chemical valve 18b are opened. [0023] Here, the chemical circulation line can also be utilized when the semipermeable membrane is subjected to circulation cleaning by means of an acid, an alkali, a detergent, etc. [0024] In the case of pressurizing and feeding a liquid containing a rejection performance enhancer of the present invention, a liquid having added thereto a rejection performance enhancer and a solute, to which the present invention is applied, may be previously prepared in the chemical tank 15, or as illustrated in Fig. 1, for example, a rejection performance enhancer and a solute may be added by the chemical dosing unit 20a and the chemical dosing unit 20b, respectively. Furthermore, it is also preferable to apply a solute contained in the semipermeable membrane feed water or semipermeable membrane concentrate as the solute of the present invention. In this case, for example, any one of semipermeable membrane feed water, semipermeable membrane concentrate and semipermeable membrane permeate is first fed and stored in the chemical tank 3 5 during fresh-water generation operation or before and after fresh¬water generation operation. Simultaneously with or after the completion of the storing work, a rejection performance enhancer is added with a predetermined concentration from the chemical dosing unit 20a. This makes it unnecessary to procure a solute/a solvent for the liquid of the present invention from outside the system and in addition, makes it possible to prepare a liquid having an osmotic pressure appropriate to the purpose. Of course, a solute may be fed from outside the system, i.e., by the chemical dosing unit 20b while using the semipermeable membrane permeate as a solvent. [0025] Intensive studies by the present inventors have revealed that, for achieving an efficient rejection performance-enhancing treatment aimed by the present invention, it is very effective to provide, in a method of enhancing a rejection performance of a semipermeable membrane by pressurizing and feeding a liquid containing a rejection performance enhancer to a primary side of the semipermeable membrane to come into contact with the membrane surface thereof, a step of changing, during the feeding of the treatment liquid, at least either the pressure acting on the membrane surface or the permeation flow rate from that at the time of normal treatment, followed by maintaining. As a specific method, the pressure or the osmotic pressure of the liquid containing the rejection performance enhancer, or the feed flow rate to the semipermeable membrane is changed at least once, whereby the object of the present invention can be achieved. [0026] The object of the present invention can be effectively achieved by changing the pressure at a rate of 0.05 MPa/s or more or changing the osmotic pressure to satisfy 0.05 MPa/s as well. [0027] Since the purpose is to change the permeation flow rate, the change in the feed flow rate must be appropriately set depending on the feeding conditions, and as to the change in the permeation flux (=permeation flow rate per membrane area), fluctuation to 0.8 limes or less or 1.2 times or more that before the change may be effective but is more preferably from 0.6 to 0.8 times or from 3.2 to 1.5 times, because rapid fluctuation places a burden on the semipermeable membrane. [0028] A method of setting the permeation flow rate to 0.1 times or less the permeation flow rate at the time of normal treatment is preferred as well. In other words, it is also effective to perform flushing of allowing the permeate to approach substantially zero. In particular, for performing the flushing, the flow is easily switched by a simple method such as opening of the concentrate side of the semipermeable membrane or full closing of the permeation side and therefore, the flushing can be easily and simply conducted, which is preferred. The time for which the pressure or permeation flow rale in the present invention is maintained after being changed is preferably from 10 seconds to 10 minutes. [0029] The number of flushings is not particularly limited, and it is also preferable to intermittently conduct the flushing while monitoring the effect. The same effect can also be obtained by the change in the permeation flow rate. In this case, the change may be achieved, as described above, by a pressure change on the primary side but can also be achieved by a pressure change on the secondary side. In addition, the permeation flux can be changed also by changing the concentration of the solute used in the present invention, thereby changing the osmotic pressure at the membrane surface. In such a case, when the raw water at the time of fresh water generation is seawater, the osmotic pressure can be greatly fluctuated without spending a cost on chemicals, which is preferred. [0030] Furthermore, for achieving an efficient rejection performance-enhancing treatment aimed by the present invention, when a constant X is determined according to the kind of the rejection performance enhancer and a quantity of the treatment liquid fed to the semipermeable membrane is denoted as QF [m3/day], a quantity of the treatment liquid permeated through the semipermeable membrane is denoted as QP |W/day], the membrane area of the semipermeable membrane is denoted as A [m~], the rejection performance enhancer concentration is denoted as C [mg/1], the liquid transit time is denoted as t [h], and the osmotic pressure of the fed liquid is denoted as n, the treatment is preferably applied to satisfy: 1.0X In implementing the preparation, the total salt concentration in permeate and feed water was determined by measuring electric conductivity of each liquid by means of an electric conductivity meter and in accordance with the relational expression of a simulated seawater concentration with an electric conductivity previously measured on simulated seawater. The simulated seawater as used herein means a liquid prepared by blending the components in a ratio of NaCl=23.926g/l, Na2SO4=4.006g/l, KCl=0.738g/l, NaHCO3=0.196g/1, MgCl2=5.072g/l, CaCI2=1.147g/l and H3BO3=0.0286g/l, and the total salt concentration when prepared at this concentration becomes 3.5 wt%. [0092] determination of Constant X> Using the flat-sheet membrane evaluation apparatus described in Non-Patent Document 2, an aromatic polyamide reverse osmosis membrane with an NaCl rejection ratio of about 99.8%, creating a permeation flux of about 1.0 [m/day] upon circulation, pressurization and permeation of an aqueous solution of 32,000 [mg/1-NaCI], 25°C and pH-7 at 55 [bar] and a feed flow rate of 3.5 [L/min], was immersed in an aqueous hypochlorous acid solution, and the rejection ratio was thereby reduced to about 99.4%. This membrane was subjected to circulation treatment with a liquid in which 2 |imol/L of polyethylene glycol having a weight average molecular weight of 8,000 was added as a rejection performance enhancer, at a flow rate of 3.5 L, 25°C and a pressure of 4.5 bar by using the same flat-sheet membrane evaluation apparatus. [0093] At this time, all of the permeate and the concentrate were cyclically utilized. After the treatment, an aqueous solution of 32,000 [mg/1-NaCl], 25°C and pH=7 was again subjected to circulation, pressurization and permeation at 55 [bar] and a feed flow rate of 3.5 [L/min], and the performance was measured to calculate a rejection performance improvement ratio [=(initial NaCl rejection ratio - NaCl rejection ratio after deterioration)/(initial NaC! rejection ratio - NaCl rejection ratio after treatment)], That is, here, the initial NaCl rejection ratio is the target rejection ratio R0. The permeation flux F after the treatment and the treatment time-dependent change of the NaCl rejection ratio R were measured, as a result, the improvement ratio exceeded 2 in 46 minutes and since F2/Fo:=0.74 at that time, the constant was determined to be X=0.77. [0094] The apparatus was operated in such a manner that, as illustrated in Fig. 10, simulated raw water prepared at a TDS concentration CF [mg/I-] in a raw water tank 2 was used, subjected to ultrafiltration through a UF membrane module manufactured by 'foray Industries, Inc. as a pretreatment unit 4 so as to prevent fouling of the semipermeable membrane, routed through a safety filter 7 by a feed pump 6 and fed to semipermeable membrane units 9a and 9b by a booster pump 8a and the obtained concentrate and permeate were totally refluxed to the raw water tank through a circulation line 13c. The semipermeable membrane units 9a and 9b each was loaded with one reverse osmosis membrane element TM8I0V manufactured by Toray Industries, Inc. and operated at an operation pressure of PF [bar], a feed flow rate of 36 [m7dayj and a temperature of 25 [°C], and the permeation flow rate QFo [m~/day] and the permeate TDS concentration CPo [mg/L] were measured. [0095] At this time, the permeate valve 17a, the valve 17c, the valve 17d and the valve 16c in Fig. 10 were fully opened, the valve 18c, the valve 16d, the permeate discharge valve 25 connected to the permeate discharge line 24, and the concentrate discharge valve 27 connected to the concentrate discharge line 26 were fully closed, and the flow rate was adjusted by controlling the concentrate valve 18a. Subsequently, the semipermeable membrane was forcedly deteriorated by adding sodium hypochlorite to the pretreatment tank to make 10 mg/L and the permeation flow rate QPI [m3/day] and the permeate TDS concentration Cpi [mg/L] were again measured using simulated raw water at the same concentration under the same operation conditions. Furthermore, after loading the semipermeable membrane unit 9a with one reverse osmosis membrane element, emptying the semipermeable membrane unit 9b, fully opening the permeate valve 17a and fully closing the permeate valve 17b, the front and rear elements were measured under the same conditions for the permeation flow rate and the permeate TDS concentration, i.e., QPti [m7day], QP]2 [m3/day], CHI [mg/L] and CP12 [mg/L]. [0096] Then, the concentration in the raw water tank 2 was adjusted to CFr [mg/L] and after adding polyethylene glycol having a weight average molecular weight of 8,000 to the raw water tank 2 to a concentration C=15 mg/L, a rejection performance-enhancing treatment was conducted for the time I by pressurization and circulation at a feed flow rate of QrT [m3/day] and a penetration flow rate of QPT [nrVday]. Thereafter, the permeation flow rate QP2 [m3/day] and the permeate TDS concentration Cp? [mg/L] were again measured using the same simulated raw water as that in the first measurement under the same operation conditions. [0097] Furthermore, after loading each of the units with one reverse osmosis membrane element, the front and rear elements were measured under the same conditions for the permeation flow rate and the permeate TDS concentration, i.e., QP2i [nr/day], QP22 [mVday], CP2! [mg/L] and CP22 [mg/L]. Here, in replacing the raw water or chemical treatment, pure water was put in the raw water tank or the chemical tank, and flushing was performed for a few minutes while fully opening the permeate discharge valve 25 and the concentrate discharge valve 27 and fully closing the valve 17a, the valve 17d and the valve 18c in order for the influence of previous raw water or chemical not to affect the next evaluation. [0098] Test results at CF=1,000 mg/L (osmotic pressure TC=0.8 bar): in Comparative Example l where Qpr/AxCxt-0.64 (1.4X) was obtained, but RIM was not enhanced, compared with Example 2. [0099] Test results at C^-l0,000 mg/L (osmotic pressure K-7.0 bar): In Comparative Example 3 where Qp/AxCxt=0.50 (<0.8X), the rejection performance enhancement ratio was RJM-1.56, revealing that the enhancement ratio is insufficient. In Comparative Example 4, the treatment was applied for a longer time than in Example 4, and QP/AxCxt=1.00 (>1.2X) was obtained, but RiM was not so much enhanced, compared with Example 2. [0100] Test results at CF=35,O0O mg/L (osmotic pressure n=24.1 bar): In Comparative Example 5 where Q,>/AxCxt=0.25 (<0.6X), the rejection performance enhancement ratio was R!M==E46, revealing that the enhancement ratio is insufficient. In Comparative Example 6, the treatment was applied for a longer time than in Example 6, and QP/AxCxt=1.00 (>X) was obtained, but RIM was not so much enhanced, compared with Example 6. [0101] CF~1,000 mg/L: Comparison results of added water dilution (=addition of 1.0 m7d) between first (front) and second (rear) semipermeable membrane elements: [0102] Here, in the case of added water dilution in the middle, when an added water dilution treatment was performed between the semipermeable membrane units 9a and 9b in Fig. 10, the treatment was performed by fully closing the valve 16c, instead fully opening the valve 16d and the valve 18c and after diluting from the diluting water line 23 and mixing and diluting in the intermediate water tank 5b, again boosting the pressure to the same pressure as that of the concentrate of the semipermeable membrane unit 9a by the booster pump 8b. [0103] In Example 7 where dilution was not performed, the entire permeate quality after the treatment was Ci>2=95 mg/1 and RiM=3.55, and the enhancement ratio was sufficient. However, while the rejection success enhancement ratio was 3.55 and was sufficient, the permeate quality when measured with one front element was CP2i=93 mg/L, and the permeate quality when measured with one rear element was CP22-74 mg/L, revealing that the treatment with the front element was greatly inferior. [0104] On the other hand, in Example 8, the permeate quality after treatment including dilution was Cp2=:105 mg/L and R\M==2.\7, and the enhancement ratio was sufficient, but there was a slight decline in permeate quality from that in Example 7. However, the permeate quality of the front element was Cp2i=91 mg/L, and the permeate quality of the rear element was Cp22=190 mg/L, revealing that the rejection performance enhancement was equivalent. [0105] In Example 9, the treatment time was increased to 35 minutes, as a result, not only the permeate quality was CP2=:97 mg/L and RIM=3.21 and the enhancement ratio was sufficient but also the permeate quality of the front element was Cs>2!=79 mg/L and the permeate quality of the rear element was Cp22=81 mg/L, revealing that the rejection performance enhancement was equivalent. [0106] Test results at CF=35,000 mg/L: The treatment was performed under the same conditions as in Example 5 except that the treated water temperature was raised to 40°C and the treatment time was decreased to 5 minutes from 6 minutes, as a result, in Example 5, CP2=:96 mg/L and RIM-3.38, and in Example 10, CP2=94 mg/L and R;M=3.75, revealing that a high treatment efficiency could be achieved in a shorter time. [0107] Pressure reduction for 15 seconds was pulsedly performed twice every 10 minutes after a continuing treatment time of 30 minutes (i.e., total treatment time including pressure reduction: 30 minutes and 30 seconds). The pressure fluctuation rate here was 0.06 MPa/s, Except for these, the treatment was performed in the same manner as in Example 2, as a result, in Example 2, CP2=2.7 mg/L, RIM~2.33 and QP2=8.25 m3/d, and in Example 11, CP2=2.8 mg/L, R1M=2.14 and QP2=8.66 mVd, revealing that reduction in the water permeation performance could be suppressed while achieving an almost equivalent rejection performance improvement. [0108] Pressure reduction for 15 seconds was pulsedly performed twice every 5 minutes after a continuing treatment time of 15 minutes (i.e., total treatment time including pressure reduction: 15 minutes and 30 seconds). The pressure fluctuation rate here was 0.06 MPa/s. Except for these, the treatment was performed in the same manner as in Example 4, as a result, in Example 4, CP2=25.0 mg/L, RIM~3.98 and QP2=8.82 m3/d, and in Example 12, CP2-25.8 mg/L, R!M=3.40 and QP2=9.53 m3/d, revealing that reduction in the water permeation performance could be suppressed while achieving an almost equivalent rejection performance improvement. [0109] Pressure reduction for 15 seconds was pulsedly performed twice every 3 minutes after a continuing treatment time of 9 minutes (i.e., total treatment time including pressure reduction: 9 minutes and 30 seconds). The pressure fluctuation rate here was 0.06 MPa/s. Except for these, the treatment was performed in the same manner as in Example 6, as a result, in Example 6, Cp2=93.0 mg/L, RJM=4.15 and Qp2=7.70 m3/d, and in Example 13, CP2=94 mg/L, R,M=3.86 and QP2-8.16 m3/d, revealing that reduction in the water permeation performance could be suppressed while achieving an almost equivalent rejection performance improvement. [0110] The treatment was performed in the same manner as in Example 11 except that simultaneously with pressure reduction for 15 seconds, the permeate valves 17a and 17b were fully closed not to allow water to flow out, as a result, in Example 11, 0^2=2.8 mg/L, RIM=2.14 and QP2=8.66 m3/d, and in Example 14, CP2=2.7 mg/L, R!M=2.28 and QP2=8.99 m /d, revealing that reduction in the water permeation performance could be suppressed while achieving an almost equivalent rejection performance improvement. [0111] The treatment was performed in the same manner as in Example 11 except that the flow into the semipermeable membrane unit was reversed by changing the connection of the feed pipeline with the permeation pipeline immediately before pressure reduction for 15 seconds, as a result, in Example 11, Ci'2^2.8 mg/L, RIM=2.14 and QP2=8.66 m3/d, and in Example 15, C[J2=2.7 mg/L, RiM=2.28 and QP2=8.99m3/d, revealing that reduction in the water permeation performance could be suppressed while achieving an almost equivalent rejection performance improvement. [0112] The results of Examples and Comparative Examples are shown together in the Tables below. The Table is large and therefore, divided into Table 1 and Table 2 but is one Table. [0115] The present invention is not limited to the embodiments described above, and a change, a modification, etc. may be appropriately made therein. In addition, the material, the shape, the dimension, the numerical value, the morphology, the number, the placement site, etc. of each constituent element in the embodiments described above are arbitrary and not limited as long as the present invention can be attained. This application is based on Japanese Patent Application No. 2015-002859 fled on January 9, 2015, the contents of which are incorporated herein by way of reference. INDUSTRIAL APPLICABILITY [0117] The present invention provides a rejection performance-enhancing method for maintaining and enhancing the performance of a semipermeable membrane used for obtaining low-concentration permeate by using raw water such as seawater, saline river water, groundwater, lake water and treated wastewater, and this method makes it possible to increase the life of a semipermeable membrane and efficiently produce fresh water at low cost. DESCRIPTION OF REFERENCE NUMERALS AND SIGNS [01181 1: Raw water line 2: Raw water tank 3: Raw water feed pump 4: Pretreatment unit 5: Intermediate water tank 6: Feed pump 7: Safety filter 8: Booster pump 9: Semipermeable membrane unit 10: Permeate line 11: Concentrate line 1 la: Concentrate discharge line I lb: Concentrated chemical line lie: Circulation line 1 Id: Concentrate line 12: Product water tank 13: Energy recovery unit 14: Chemical feed line 15: Chemical tank 3 6a: Feed water valve 16b: Feed chemical valve 16c: Valve 16d: Valve 17a: Permeate valve 17b: Permeated chemical valve 17c: Valve 17d: Valve 18a: Concentrate valve 18b: Concentrated chemical valve 18c: Concentrate valve 18d: Valve 19: Chemical feed pump 20: Chemical dosing unit 21: Diluting water tank 22: Diluting water feed pump 23: Diluting water line 24: Permeate discharge line 25: Permeate discharge valve 26: Concentrate discharge line 27: Concentrate discharge valve 28a: Valve 28b: Valve CLAIMS [Claim !] A semipermeable membrane rejection performance-enhancing method which is a method of enhancing a rejection performance of a semipermeable membrane by pressurizing and feeding a liquid containing a rejection performance enhancer to a primary side of the semipermeable membrane to come into contact with a membrane surface thereof, the method comprising: a step of changing, at least once during the feeding, a pressure or an osmotic pressure of the liquid containing the rejection performance enhancer at a fluctuation rate of 0.05 MPa/s or more, or a feed flow rate to the semipermeable membrane, thereby changing at least either a pressure acting on the membrane surface or a permeation flow rate from that at the time of normal treatment, followed by maintaining. [Claim 2] The semipermeable membrane rejection performance-enhancing method according to claim 1, wherein a time for which the pressure or the permeation flow rate is maintained is from 10 seconds to 10 minutes. [Claim 3] The semipermeable membrane rejection performance-enhancing method according to claim 1 or 2, wherein the permeation flow rate is fluctuated to 0.8 times or less or 1.2 times or more the permeation flow rate at the time of normal treatment, at least once during the feeding. [Claim 4] The semipermeable membrane rejection performance-enhancing method according to any one of claims 1 to 3, wherein the change of the permeation flow rate is caused by a pressure change at least on the primary side or a secondary side of the semipermeable membrane. [Claim 5] The semipermeable membrane rejection performance-enhancing method according to any one of claims 1 to 4, wherein the permeation flow rate is set to 0.1 times or less the permeation flow rate at the time of normal treatment. [Claim 6] The semipermeable membrane rejection performance-enhancing method according to any one of claims 1 to 5, wherein a feed direction to the semipermeable membrane is reversed at least once. [Claim 7] The semipermeable membrane rejection performance-enhancing method according to any one of claims 1 to 6, wherein the liquid containing the rejection performance enhancer contains a solute different from the rejection performance enhancer, and when a constant X is determined according to the kind of the rejection performance enhancer and a quantity of the treatment liquid fed to the semipermeable membrane is denoted as QFT [m7day], a quantity of the treatment liquid permeated through the semipermeable membrane is denoted as QPT [m3/day], a membrane area of the semipermeable membrane is denoted as A [nr], a rejection performance enhancer concentration is denoted as C [mg/1], a liquid transit time is denoted as t [h], and an osmotic pressure of the fed liquid is denoted as n, the treatment is applied to satisfy: 1.0X