PURPOSE: To make suction to a cylinder performable with supercharging pressure commensurate to a volumetric variation in an air charging chamber with a vertical motion of a piston, by forming the air charging chamber to be comparted by a partition wall installed interposingly between the cylinder and a crankcase, in the underside of a skirt part of the piston.

CONSTITUTION: An intermediate rod 10, extending downward in the axial direction of a cylinder and being connected to a cone rod 4, is hangingly installed in a piston 3, while a partiton wall 11 into which the intermediate rod 10 is slidably inserted is installed interposingly between the cylinder and a crankcase 2, and an air charging chamber 12 is formed up. And, the air charging chamber 12 is made to interpose in a suction passage 13 of the cylinder, and each check valve is installed in both inlet and outlet sides of the passage 13. With this constitution, an air-fuel mixture out of an ustream part of the suction passage 13 leading to a carburetor is fed to a downstream part 13b of the suction passage 13 leading to a suction valve 7 through the check valve, the air-charging chamber 12 and a check valve 15. And, the supercharging pressure required is secured by a volumetric vibration in the air-charging chamber 12 with a vertical motion of the piston 3.