To provide a method for producing an N-chlorosulfonyl--lactam compound in a higher yield as compared to those in conventional methods, through solving the problem that the reaction between a butenyl compound and chlorosulfonyl isocyanate has proceeded in lower yield so far.

The N-chlorosulfonyl--lactam compound can be prepared in a higher yield compared to that in conventional methods, by conducting the reaction between a butenyl compound and chlorosulfonyl isocyanate in the presence of a cycloalkane solvent. The corresponding -lactam compound can be prepared in high yield by reducing thus obtained N-chlorosulfonyl--lactam compound.