PURPOSE: To perform rapid transfer of mixed combustion gas to a main combustion chamber and to reduce the size of a structure.

CONSTITUTION: At the second half stage of a compression stroke, the injection hole 32 of a fuel gas chamber 31 is inserted in the outlet 26 of an auxiliary chamber 21 and combustion gas is injected from the fuel gas chamber 31 to the auxiliary chamber 21. Consecutively, combustion gas in the auxiliary chamber 21 is injected to the outer peripheral area of the main combustion chamber 22 through a communication hole 23. Combustion in the main combustion chamber 22 is started from the outer periphery thereof and combustion gas gathers to the center of the main combustion chamber 22. A combustion time is shortened and the combustion efficiency of an engine is improved. Further, since the auxiliary chamber 21 is arranged in a piston 2, the structure of a cylinder head 3 is simplified and the degree of freedom of the design of an engine is increased.