A process for producing a compound represented by the following formula (II) which comprises treating a compound represented by the following formula (I) (wherein R.sup.1, R.sup.2 and R.sup.3 represent each a specific substituent) with an enzyme capable of asymmetrically hydrolyzing an ester and the novel compound (II). The process of the present invention makes it possible to easily obtain an optically active 2-azetidinone derivative in a large amount at a low cost ##STR00001##