PURPOSE: To prevent combustion noises and vibrations of an engine by increasing the temperature of the intake air in an operation chamber and shortening the ignition delay time during the fuel injection at a low-load region.

CONSTITUTION: An intake valve 5 is constituted so as to be closed at the bottom dead point of a piston 1 during an intake stroke, and a sub-intake valve 7 is constituted so as to start opening immediately before the intake valve 5 is closed and to be closed in the middle of the compression stroke. During the supercharging of an engine, the intake air fed by pressure from a supercharger 10 is supercharged into an operation chamber 3 in addition to the intake air poured into the operation chamber 3 through an intake air passage 8. At a low- load region, a control valve 9 is fully closed and closing valves 20, 23 are fully opened. The intake air flowing into an operation space 15 is discharged into a communicating passage 19 and the discharge side passage 17 of the supercharger 10 and is poured into the operation chamber 3 of the engine.