in the invention , the molar ratio of optically active 1 -( p - tolyl ) ethylamine as a resolving agent to (±)- 2 - hydroxy - 4 - phenylbutanoic acid is not particularly restricted , but it is preferable to use the amine in an amount of 0 . 4 to 1 equivalent based on (±)- 2 - hydroxy - 4 - phenylbutanoic acid so that (±)- 2 - hydroxy - 4 - phenylbutanoic acid may be resolved in good efficiency and in a high purity . this 1 -( p - tolyl ) ethylamine is generally used in the presence of a solvent , and solvents usable for this purpose include , for example , c 1 - 6 , preferably c 1 - 4 alkanols such as methanol , ethanol , 2 - propanol , 1 - propanol and 1 - butanol , c 3 - 6 alkyl methyl ketones such as acetone and methyl isobutyl ketone , unsubstituted or methyl - or ethyl - substituted benzenes such as benzene , toluene and xylene , c 6 - 8 cycloalkanes such as cyclohexane , c 6 - 10 alkanes such as n - hexane , n - heptane , n - octane and n - decane , water , dioxane , tetrahydrofuran and tetrahydropyran and mixtures thereof . above all , water dioxane and methyl isobutyl ketone are preferable as they can give an optically active 2 - hydroxy - 4 - phenylbutanoic acid of a high purity . in case of using water as the solvent , it si preferable to neutralize the excess amount of (±)- 2 - hydroxy - 4 - phenylbutanoic acid with a base such as sodium hydroxide , potassium hydroxide , lithium hydroxide or ammonia because an optically active 2 - hydroxy - 4 - phenylbutanoic acid of high purity can be obtained . the amount of a solvent to be used is varied depending on kind of solvents , dissolution temperature or crystallization temperature and cannot be definitely prescribed , but may usually be selected from the range of 150 to 1500 ml per mole of 1 -( p - tolyl ) ethylamine . preferred crystallization temperature is also varied depending on the amount of a solvent to be used , kind of solvent or dissolution temperature , but may usually be selected from the range of - 10 ° to 50 ° c . from an economical view . in case of using an optically active n -( 2 - hydroxy ) ethyl - α - methylbenzylamine as the optical resolving agent , the molar ratio of the amine to (±)- 2 - hydroxy - 4 - phenylbutanoic acid is not particularly restricted . however , it is preferable to use the amine in an amount of 0 . 4 to 1 . 0 equivalent based on (±)- 2 - hydroxy - 4 - phenylbutanoic acid . n -( 2 - hydroxy ) ethyl - α - methylbenzylamine is generally used in the presence of a solvent , and solvents usable for this purpose include , for example , c 1 - 6 , preferably c 1 - 4 alkanols such as methanol , ethanol , 2 - propanol , 1 - propanol and 1 - butanol , c 3 - 6 alkyl methyl ketones such as acetone and methyl isobutyl ketone , unsubstituted or methyl - or ethylsubstituted benzenes such as benzene , toluene and xylene , c 6 - 8 cycloalkanes such as cyclohexane , c 6 - 10 alkanes such as n - hexane , n - heptane , n - octane and n - decane , water , dioxane , tetrahydrofuran and tetrahydropyran and mixtures thereof . above all , unsubstituted or methyl - or ethylsubstituted benzenes , c 6 - 8 cycloalkanes , c 6 - 10 alkanes , and mixtures thereof , and water are preferable because they can give an optically active 2 - hydroxy - 4 - phenylbutanoic acid of high purity . in case of using water as the solvent , it is preferable to neutralize the excess amount of (±)- 2 - hydroxy - 4 - phenylbutanoic acid with a base such as sodium hydroxide , potassium hydroxide , lithium hydroxide or ammonia because an optically active 2 - hydroxy - 4 - phenylbutanoic acid of high purity can be obtained . the amount of a solvent to be used is varied depending on kind of solvents , dissolution temperature or crystallization temperature and cannot be definitely prescribed , but may usually be selected from the range of 150 to 1500 ml per mole of n -( 2 - hydroxy ) ethyl - α - methylbenzylamine . preferred crystallization temperature is also varied depending on the amount of a solvent to be used , kind of the solvent or dissolution temperature , but may usually be selected from the range of - 20 ° to 50 ° c . from an economical view . the process of the present invention is carried out , for example , in the following manner . (±)- 2 - hydroxy - 4 - phenylbutanoic acid and 0 . 4 to 1 equivalent , based on the (±)- 2 - hydroxy - 4 - phenylbutanoic acid , of optically active 1 -( p - tolyl ) ethylamine or n -( 2 - hydroxy ) ethyl - α - methylbenzylamine are added to a solvent , followed by heating for dissolution . subsequently , the solution is cooled to supersaturation . preferably , a (±)- 2 - hydroxy - 4 - phenylbutanoic acid /(-)- 1 -( p - tolyl ) ethylamine salt , (-)- 2 - hydroxy - 4 - phenylbutanoic acid /(±)- 1 -( p - tolyl ) ethylamine salt , (±)- 2 - hydroxy - 4phenylbutanoic acid /(±)- n -( 2 - hydroxy ) ethyl - α - methylbenzylamine salt or (-)- 2 - hydroxy - 4 - phenylbutanoic acid /(-)- n -( 2 - hydroxy ) ethyl - α - methylbenzylamine salt containing 1 -( p - tolyl ) ethylamine or n -( 2 - hydroxy ) ethyl - α - methylbenzylamine in the same optically active form as that of the starting amine is added in a small amount , thereby permitting the same kind of sparlingly soluble diastereomer salt to be deposited , followed by separation of this salt . the separation of the diastereomer salt may be effected by a method such as filtration or centrifugation . the thus obtained diastereomer salt is treated with a base such as sodium hydroxide , potassium hydroxide or sodium methoxide , and (-) or (±) 1 -( p - tolyl ) ethylamine or (-) or (±) n -( 2 - hydroxy ) ethyl - α - methylbenzylamine is recovered , followed by further treatment with an acid such as hydrochloric acid , sulfuric acid , phosphoric acid or p - toluenesulfonic acid to obtain (±) or (-) 2 - hydroxy - 4 - phenylbutanoic acid . the above procedures may alternatively be carried out in the order of the acid treatment and optional subsequent base treatment . 0 . 36 g ( 2 mmol ) of (±)- 2 - hydroxy - 4 - phenylbutanoic acid ( hereinafter abbreviated as (±)- 1 ) and 0 . 27 g ( 2 mmol ) of (±)- 1 -( p - tolyl ) ethylamine ( hereinafter abbreviated as (±)- 2 ) were added to 3 . 5 ml of dioxane and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 0 . 18 g ( 0 . 58 mmol ) of (-)- 1 /(±)- 2 salt . the yield based on the employed (-)- 1 was 58 . 0 %, m . p . = 148 °- 153 ° c . and [ α ] 589 =+ 12 . 9 ° ( c = 1 . 1 , methanol ). 0 . 7 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 0 . 9 ml of 1n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to distil away the solvent , whereby 0 . 10 g ( 0 . 57 mmol ) of (-)- 1 was obtained . yield 57 . 0 %, m . p . = 109 °- 112 ° c ., [ α ] 589 =- 7 . 01 ° ( c = 0 . 9 , 99 % ethanol ), optical purity 80 . 6 %. 1 . 08 g ( 10 mmol ) of (±)- 1 and 1 . 08 g ( 8 mmol ) of (±)- 2 were added to 8 ml of dioxane and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 0 . 97 g ( 3 . 08 mmol ) of (-)- 1 /(±)- 2 salt . the yield based on the employed (-)- 1 was 61 . 6 %, m . p . = 145 °- 149 ° c ., [ α ] 589 =+ 11 . 1 ° ( c = 1 . 1 , methanol ). this salt was recrystallized from 2 . 7 ml of dioxane to obtain 0 . 80 g ( 2 . 53 mmol ) of (-)- 1 /(±)- 2 salt . the yield based on the employed (-)- 1 was 50 . 6 %, m . p . = 149 °- 150 ° c ., [ α ] 589 =+ 13 . 5 ° ( c = 1 . 0 , methanol ). 3 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 4 ml of 1n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to distil away the solvent , whereby 0 . 45 g ( 2 . 51 mmol ) of (-)- 1 was obtained . yield 50 . 2 %, m . p . = 111 °- 114 ° c ., [ α ] 589 =- 8 . 21 ° ( c = 1 . 1 , 99 % ethanol ), 5 . 41 g ( 30 mmol ) of (±)- 1 and 3 . 25 g ( 24 mmol ) of (±)- 2 were added to 24 ml of dioxane and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 3 . 09 g ( 9 . 80 mmol ) of (-)- 1 /(+)- 2 salt . the yield based on the employed (-)- 1 was 65 . 3 %. m . p . = 145 °- 148 ° c ., [ α ] 589 =+ 11 . 5 ° ( c = 1 . 5 , methanol ). this salt was recrystallized from 8 . 7 ml of dioxane to obtain 2 . 60 g ( 8 . 23 mmol ) (-)- 1 /(+)- 2 salt . the yield based on the employed (-)- 1 was 54 . 9 %. m . p . = 149 °- 153 ° c ., [ α ] 589 =+ 12 . 7 ° ( c = 1 . 2 , methanol ). 9 . 6 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extractior . 1 . 9 ml of 6n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to distil away the solvent , whereby 1 . 47 g ( 8 . 17 mmol ) of (-)- 1 was obtained . yield 54 . 5 %, m . p . = 111 °- 113 ° c ., [ α ] 589 =- 8 . 27 ° ( c = 1 . 2 , 99 % ethanol ), optical purity 95 . 1 %. 0 . 60 g ( 2 mmol ) of (±)- 1 and 0 . 70 g ( 2 mmol ) of (±)- 2 were added to 1 . 3 ml of methyl isobutyl ketone and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 0 . 24 g ( 0 . 76 mmol ) of (-)- 1 /(+)- 2 salt . this salt was recrystallized from 1 . 5 ml of methyl isobutyl ketone to obtain 0 . 17 g ( 0 . 55 mmol ) of (-)- 1 /(±)- 2 salt . the yield based on the employed (-)- 1 was 55 . 0 %, m . p . = 146 °- 149 ° c ., [ α ] 589 =+ 13 . 0 ° ( c = 1 . 2 , methanol ). 1 . 5 ml of a 1n sodium hydroxide solution was added to the salt and the mixture was extracted with ether . 0 . 7 ml of 1n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to distil away the solvent , whereby 0 . 10 g ( 0 . 53 mmol ) of (-)- 1 was obtained . yield 53 . 0 %, m . p . = 108 °- 111 ° c ., [ α ] 589 =- 8 . 28 ° ( c = 0 . 9 , 99 % ethanol ), optical purity 95 . 2 %. 1 . 80 g ( 10 mmol ) of (±)- 1 and 1 . 08 g ( 8 mmol ) of (±)- 2 were added to 6 ml of methyl isobutyl ketone and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 1 . 53 g ( 4 . 85 mmol ) of (-)- 1 /(+)- 2 salt . this salt was recrystallized from 7 . 0 ml of methyl isobutyl ketone to obtain 1 . 05 g ( 3 . 33 mmol ) of (-)- 1 /(+)- 2 salt . the yield based on the employed (-)- 1 was 66 . 6 %. m . p . = 143 °- 151 ° c ., [ α ] 589 =+ 12 . 9 ° ( c = 1 . 0 , methanol ). 4 . 0 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 5 . 0 ml of 1n hydrochloric acid was added to the resulting water layer , followed by ether extraction . the organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to distil away the solvent , whereby 0 . 54 g ( 3 . 00 mmol ) of (-)- 1 was obtained . yield 60 . 0 %, m . p . = 102 °- 108 ° c ., [ α ] 589 =- 6 . 61 ° ( c = 1 . 1 , 99 % ethanol ), optical density 76 . 0 %. 1 . 80 g ( 10 mmol ) of (±)- 1 - and 1 . 08 g ( 8 mmol ) of (±)- 2 were added to 8 ml of methyl isobutyl ketone and heated to form a solution , which was then gradually cooled to room temperature . after standing overnight , the deposited crystals were separated by filtration to obtain 1 . 16 g ( 3 . 67 mmol ) of (-)- 1 /(+)- 2 salt . this salt was recrystallized from 5 . 3 ml of methyl isobutyl ketone to obtain 0 . 95 g ( 3 . 00 mmol ) of (-)- 1 /(±)- 2 salt . the yield based on the employed (-)- 1 was 60 . 0 %. m . p . = 147 °- 150 ° c ., [ α ] 589 =+ 14 . 0 ° ( c = 1 . 5 , methanol ). 3 . 6 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 4 . 3 ml of 1n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and the solvent was distilled away under reduced pressure to obtain 0 . 53 g ( 2 . 96 mmol ) of (-)- 1 . yield 59 . 2 %, m . p .= 110 °- 113 ° c ., [ α ] 589 =- 7 . 17 ° ( c = 2 . 0 , 99 % ethanol ), optical purity 82 . 4 %. 450 g ( 2 . 5 mol ) of (±)- 1and 270 g ( 2 . 0 mol ) of (±)- 2 were added to 2000 ml of dioxane and heated to form a solution , which was then gradually cooled to room temperature . after standing for 24 hours , the deposited crystals were separated by filtration . these crystals were heated again with 700 ml of dioxane . the resulting solution was gradually cooled to room temperature and the deposited crystals were separated by filtration . this procedure was once repeated to obtain 313 . 9 g ( 0 . 997 mol ) of (-)- 1 /(+)- 2 salt . the yield based on (-)- 1 was 79 . 8 %. m . p . 153 ° c . 1200 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 1000 ml of 1n hydrochloric acid was added to the resulting aqueous layer , followed by ether extraction . the organic layer was dried over anhydrous sodium sulfate and ether was distilled away to obtain 174 . 72 g ( 0 . 971 mol ) of (-)- 1 salt . yield 77 . 7 %, [ α ] 589 =- 8 . 61 ° ( c = 1 . 0 , 99 % ethanol ), optical purity 99 %. 25 . 9 g ( 0 . 144 mol ) of (±)- 1 and 19 . 0 g ( 0 . 115 mol ) of (-)- n -( 2 - hydroxyethyl )- α - methylbenzylamine ( hereinafter abbreviated as (-)- 3 ) were added to a mixed solvent of 74 ml of benzene and 31 ml of hexane and heated to form a solution . the solution was gradually cooled to room temperature and allowed to stand for 24 hours , and the deposited crystals were separated by filtration . these crystals were recrystallized from a mixed solvent of 56 ml of benzene and 20 ml of hexane and further from a mixed solvent of 91 ml of benzene and 39 ml of hexane to obtain 15 . 56 g ( 0 . 045 mol ) of (-)- 1 /(-)- 3 salt . m . p . 101 °- 102 ° c . the yield based on the employed (-)- 1 was 63 %. 54 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . 72 ml of 1n hydrochloric acid was added to the resulting aqueous layer , which was then extracted with ether . the organic layer was dried over anhydrous sodium sulfate and ether was distilled away to obtain 7 . 86 g ( 0 . 0437 mol ) of (-)- 1 . yield 61 %, [ α ] 589 =- 8 . 58 ° ( c = 1 . 0 , 99 % ethanol ), optical purity 99 %. 18 . 0 g ( 0 . 1 mol ) of (±)- 1 , 8 . 1 g ( 0 . 06 mol ) of (±)- 2 and 1 . 6 g ( 0 . 04 mol ) of sodium hydroxide were dissolved in 50 g of water at 72 ° c . after being allowed to cool overnight , the deposited crystals were separated by filtration to obtain 11 . 9 g ( 0 . 038 mol ) of (-)- 1 /(+)- 2 salt . the yield based on the used (-)- 1 was 75 %. 40 ml of 1n sodium hydroxide solution was added to the salt , followed by ether extraction . the ether layer was dried over anhydrous sodium sulfate and the solvent as distilled away under reduced pressure to obtain 6 . 3 g ( 0 . 035 mol ) of (-)- 1 . the hplc analysis of this compound gave (-)- 1 /(+)- 1 = 98 . 5 / 1 . 5 . optical purity 97 . 0 %. 18 . 0 g ( 0 . 10 mol ) of (±)- 1 , 8 . 1 g ( 0 . 06 mol ) of (±)- 2 and 2 . 7 g ( 0 . 04 mol ) of 25 % ammonia water were dissolved in 50 g of water at 80 ° c . after being allowed to cool overnight , the deposited crystals were separated by filtration to obtain 13 . 0 g ( 0 . 041 mol ) of (-)- 1 /(±) 2 salt . this salt was twice recrystallized from each 50 g of water to obtain 8 . 6 g ( 0 . 027 mol ) of (-)- 1 /(+)- 2 salt . the yield based on (-)- 1 was 55 %. 40 ml of a 1n sodium hydroxide solution was added to the salt , followed by ether extraction . the ether layer was dried over anhydrous sodium sulfate and the solvent was distilled away under reduced pressure to obtain 4 . 5 g ( 0 . 025 mol ) of (-)- 1 . the analysis of this compound by hplc under the same condition as in example 9 gave (-)- 1 /(+)- 1 = 99 . 5 / 0 . 5 . optical purity 99 . 0 %. 18 . 0 g ( 0 . 1 mol ) of (±)- 1 , 6 . 7 g ( 0 . 05 mol ) of (+)- 2 and 2 . 0 g ( 0 . 05 mol ) of sodium hydroxide were dissolved in 50 g of water at 67 ° c . after being allowed to cool overnight , the deposited crystal were separated by filtration to obtain 13 . 0 g ( 0 . 041 mol ) of (-)- 1 /(+)- 2 salt . the yield based on the used (-)- 1 was 82 %. 40 ml of 1n sodium hydroxide solution was added to the salt , followed by ether extraction . the ether layer was dried over anhydrous sodium sulfate and the solvent was distilled away under reduced pressure to obtain 7 . 0 g ( 0 . 039 mol ) of (-)- 1 . yield 78 %. the hplc analysis of this compound under the same conditions as in example 9 gave (-)- 1 /(±)- 1 = 99 . 0 / 1 . 0 . optical purity 98 . 0 %. 18 . 0 g ( 0 . 1 mol ) of (±)- 1 , 6 . 7 g ( 0 . 05 mol ) of (+)- 2 and 2 . 0 g ( 0 . 05 mol ) of sodium hydroxide were dissolved in 30 g of water at 68 ° c . after being allowed to cool overnight , the deposited crystals were separated by filtration to obtain 10 . 7 g ( 0 . 034 mol ) of (-)- 1 /(+)- 2 salt . the yield based on the used (-)- 1 was 68 %. 40 ml of 1n sodium hydroxide solution was added to the salt , followed by ether extraction . the ether layer was dried over anhydrous sodium sulfate and the solvent was distilled away under reduced pressure to obtain 5 . 8 g ( 0 . 032 mol ) of (-)- 1 . yield 64 %. the hplc analysis of this compound under the same conditions as in example 9 gave (-)- 1 /(+)- 1 = 98 . 8 / 1 . 2 . optical purity 97 . 6 %. 9 . 0 g ( 0 . 05 mol ) of (±)- 1 , 4 . 95 g ( 0 . 03 mol ) of (±)- 3 and 0 . 8 g ( 0 . 02 mol ) of sodium hydroxide were dissolved in 25 g of water at 50 ° c . after being allowed to cool overnight , the deposited crystals were separated by filtration to obtain 7 . 68 g ( 0 . 022 mol ) of (-)- 1 /(-)- 3 salt . the yield based on the used (-)- 1 was 88 %. 20 ml of 1n sodium hydroxide solution was added to the salt , followed by ether extraction . the ether layer was dried over anhydrous sodium sulfate and the solvent was distilled away under reduced pressure to obtain 3 . 6 g ( 0 . 02 mol ) of (-)- 1 . yield 80 %. the hplc analysis of this compound under the same conditions as in example 9 gave (-)- 1 /(+)- 1 = 98 . 9 / 1 . 1 . optical purity 97 . 8 %.