PURPOSE: To absorb and alleviate pulsation of each cylinder by an expansion chamber, by a method wherein the expansion chamber is formed downstream a supercharger in an auxiliary suction duct and the upper end of a stream of an auxiliary suction port of each cylinder is connected with the expansion chamber.

CONSTITUTION: Sucked air by natural suction is fed to a main suction port 5 of each cylinder 1 by a main suction duct 11. An auxiliary suction port 7 is opened during a period from the termination of suction process to the beginning of compression process. A rotary shaft 19a of a supercharger 19 is driven by a timing belt 23 through a solenoid clutch 22. An expansion chamber 26 having a large capacity is provided downstream an auxiliary throttle valve 20 of an auxiliary suction duct 18, the upper end of the auxiliary suction port 7 of each cylinder 1 is connected with the expansion chamber 26 and a pulsation of a combustion chamber 4 side which becomes active when the auxiliary suction port 7 is opened by an auxiliary suction valve 8 is eliminated by the expansion chamber 26.