PURPOSE: To convert the 2-cycle and the 4-cycle easily by providing main and sub combustion chambers linked through a linking passage, furnishing a switching means to the linking passage, and opening and closing the switching means at every alternate compression stroke in the 4-cycle operation, while stopping the switching means in the opening condition in the 2-cycle operation.

CONSTITUTION: A suction port 11 is formed at the lower side of a cylinder 1, and an exhaust port 12 is formed at the upper position of a specific amount from the position of the suction port 11. The suction port 11 is branched to a suction pipe 13 just before linking to the cylinder 1, the suction pipe 13 is opened at the upper end surface of the cylinder 1, and a suction air valve 14 is provided to the opening. A sub-combustion chamber 3 linked to a main combustion chamber 21 which is partitioned by the inner wall of the cylinder 1 and a piston 2, through a linking passage 31, is formed in a cylinder head, and an injection nozzle 33 is provided at the inside of the sub-combustion chamber 3, while a sub-chamber valve 32 is provided to the linking passage 31. And the sub-chamber valve 32 is opened and closed at every alternate compression stroke in the 4-cycle operation, while it is stopped in the opening condition in the 2-cycle operation.