To obtain the subject optically active compound especially useful as an intermediate for a β-lactam compound by reducing 4-sulfenyl-substituted acetoacetic acid ester with baker's yeast in the presence of a specific compound, thereby proceeding the reaction in a short time.

An optically active 3-hydroxy-4-sulfenyl-substituted acetoacetic acid ester of the formula II is produced by reducing a 3-oxo-4-sulfenyl-substituted acetoacetic acid ester of the formula I (R1 is phenyl or a phenylalkyl; R2 is an alkyl) with baker's yeast in the presence of saccharose. A reduction product can be produced in high enanthio-selectivity by the reduction of an acetoacetic acid ester derivative having sulfenyl group at the 4-site. Especially, an optically pure reduction product of R isomer is produced by using ethyl 4- phenylthioacetoacetate. The reduction product can be converted into β- hydroxyamide and induced to a β-lactam compound by photo-extension reaction.