PURPOSE: To correct the degree of vacuum on suction side of a supercharger end to prevent lowering of a supercharger capacity by connecting a vacuum compensating pipe branching before a throttle valve to a casing of a supercharging pipe.

CONSTITUTION: Vacuum is produced between a throttle valve F and a suction opening port 2 of a supercharger X. Compensating air is sucked in the interior of a casing of the supercharger X by a vacuum compensating pipe 21 not related to the opening of the throttle valve F to compensate the degree of vacuum of the suction opening port 2. The interior of the casing of the supercharger X is corrected to have substantially equal pressure to the outside air pressure to correct the degree of vacuum of an air feed pipe 23 and a manifold 24 when a piston P takes suction operation, whereby the tendency of an engine brake is relaxed.