This invention provides a process for preparing a .beta.-lactam derivative represented by the formula ##STR1## wherein R.sup.1 represents aralkyl group or aryloxy methyl group, Y represents the group ##STR2## (wherein R.sup.2 represents aryl-substituted lower alkyl group, aryloxy-substituted lower alkyl group or lower alkyl group optionally substituted with halogen atom and X represents halogen atom) or the group ##STR3## (wherein R.sup.2 and X are as defined above), the process comprising electrolyzing an azetidinone derivative of the formula ##STR4## wherein R.sup.1 and R.sup.2 are as defined above in the presence of hydrohalogenic acid and/or halide.