A method which enables olefin compound production with a high productivity and an enzyme used in the method, a mutation involving amino acid substitution has been introduced into various sites of diphosphomevalonate decarboxylase (MVD), thus preparing a large number of MVD variants. Next, the result of evaluating the variants for the catalytic activity related to the production of olefin compounds such as isoprene has revealed that MVD whose threonine at position 209 is substituted with a different amino acid has the catalytic activity, and that MVD whose arginine at position 74 is further substituted with a different amino acid in addition to position 209 has the catalytic activity at higher levels.