To improve the efficiency of an engine under a low load.

A sliding valve body 6 is provided in an auxiliary cylinder 2 and an auxiliary combustion chamber 7 is provided in the sliding valve body 6. A first compression ring 8 and a second compression ring 12 are provided on the inner wall of the auxiliary cylinder 2. An opening 11 is provided in a portion of the auxiliary cylinder 2 between the two compression rings 8 and 12. A drive device 9 is provided for moving the sliding valve body 6. Under a high load, the drive device 9 moves the sliding valve body 6 to a position where the opening 11 of the auxiliary cylinder 2 faces an opening 10 of the auxiliary combustion chamber 7. Then, the auxiliary combustion chamber 7 communicates with a main combustion chamber 1.