A method for producing an α-L-aspartyl-L-phenylalanine-β-ester (also named α-L-β-o-substituted aspartyl)-L-phenylalanine), which is an intermediate of an α-L-aspartyl-L-phenylalanine-α-methyl ester (also named a-L-aspartyl-L-phenylalanine methyl ester product name: aspartame), easily, at high yield and inexpensively without going through a complex synthesis method is provided. Further, an easy, inexpensive and high-yield production method for an α-L-aspartyl-L-phenylalanine-α-methyl ester is provided. α-L-aspartyl-L-phenylalanine-a-methyl ester is produced from a L-aspartic acid-α,β-diester and L-phenylalanine using an enzyme or enzyme-containing substance that has an ability to catalyze a reaction in which L-phenylalanine performs no nucleophilic attack on a β-ester site of L-aspartic acid-α,β-diester but performs a nucleophilic attack on an α-ester site thereof.
