Abstract: Disclosed is a process for producing an optically active .beta.-lactone represented by the general formula (I): ##STR1## wherein R.sup.1 represents an alkyl group having 1-9 carbon atoms which can be branched or a cycloalkyl group having 5-7 carbon atoms. The process comprises reacting an aldehyde represented by the general formula (II):R.sup.1 CHO (II)wherein R.sup.1 is the same as defined above, with a ketene represented by the general formula (III):CH.sub.2 .dbd.C.dbd.O (III)in the presence of a bissulfonamide/aluminum complex represented by the general formula (IV): ##STR2## wherein Ar represents 2,4,6-(R.sup.2).sub.3 C.sub.6 H.sub.2 (wherein R.sup.2 represents an alkyl group having 2-4 carbon atoms), Et represents an ethyl group and Ph represents a phenyl group.