To prepare a desired enantiomer of the subject compound in a high yield and/or a high diastereomeric ratio by bringing a specific aldehyde into contact with a specified optically active acetyl compound in the presence of a base.

An aldehyde represented by formula I (R is phenyl) is brought into contact with an optically active acetyl compound represented by formula II (X is an eliminable group; A is derived from an amino-alcohol which is an optically active compound) in the presence of a base (preferably potassium t-butoxide) to thereby provide an optically active trans-phenylglycidic acid derivative represented by formula III. An aminoindanol compound represented by formula II [A is represented by formula IV [R1 and R2 are each a 1-10C (hetero)alkyl, a 1-10C alkenyl, a 1-10C (hetero)aryl, etc.] is preferred in the compounds of formula II. The compound represented by formula III obtained thereby can advantageously be used for producing a medicine, especially benzothiazepine, e.g. diltiazem and clenthiazem.