The present invention provides a catalyst which exhibits a high catalytic activity and allows asymmetric reaction in a high asymmetric yield to obtain a reaction product having a high optical purity. The present invention also provides a process for the preparation of an optically active 4-methyl-2-oxetanone having a high optical purity useful as starting material of polymer in a high yield in a short period in the presence of the foregoing catalyst. A novel ruthenium-iodo-optically active phosphine complex is provided, represented by the following general formula (1):Ru--I.sub.2 --(R.sup.1 -BINAP) (1)wherein R.sup.1 -BINAP represents an optically active tertiary phosphine represented by the following general formula (2): ##STR1## wherein R.sup.1 represents an aryl group which may have a substituent selected from the group consisting of a C.sub.1-4 lower alkyl group, a C.sub.1-4 lower alkoxy group, a C.sub.1-4 lower alkylamino group, and a halogen atom or a C.sub.3-8 cycloalkyl group.