To selectively produce optically active carboxylic acids of high optical purity useful in industrial fields such as medicines, agrochemicals, perfumes, food additives and electronics in high yield.

An optically active 4,5-diphenyl-2-imidazolidinone derivative represented by formula II (R1 is an alkyl, etc.; R2 is a dialkoxyphosphoryl or a halogen; * indicates the position of asymmetric carbon) is reacted with an aldehyde represented by the formula R3CHO (R3 is an organic group) to afford an α,β-unsaturated acyl compound represented by formula III. The Michael addition reaction or Michael type addition reaction of the resultant α,β-unsaturated acyl compound represented by formula III is then carried out by using a reagent having an organic group R4 different from R3 to provide a diastereomer represented by formula IV (R4 is the organic group different from R3), which is then hydrolyzed to afford optically active carboxylic acids represented by formula I, e.g. (4S,5S)-1-cinnamoyl-4,5-diphenyl-3-methyl-2-imidazolidinone.