PURPOSE: To restrict a maximum combustion pressure to a lower value by providing an auxiliary chamber, which contains a movable piston, on a cylinder head to communicate with a combustion chamber.

CONSTITUTION: When a pressure in a combustion chamber 4 is lower than a specific value, a piston 8 inside an auxiliary chamber 6 is pushed down by a spring 9 until the bottom face of the piston 8 becomes a part area of the peripheral wall of a combustion chamber 4. During a combustion stroke, when the pressure in the combustion chamber 4 rises over a force of the spring 9 applied to the movable piston 8, the movable piston 8 is forced up inside the auxiliary chamber 6. As a capacity of the combustion chamber may be increased, a maximum combustion pressure can be maintained low.