PURPOSE: To ignite mixture gas in a reliable manner and to stabilize combustion of the mixture gas, by forming the entire inner surface of a cylinder head substantially in a flat plane, and forming a substantially flat area and a recess constituting a combustion chamber in the top surface of a piston.

CONSTITUTION: The entire inner surface 2A of a cylinder head 2 is formed to be substantially flat. A squish area 14 is formed between the inner surface 2A of the cylinder head 2 and a substantially flat area 10A of a piston 10, while a combustion chamber 15 is formed between the inner surface 2A and a recess 10B formed in the top surface of the pisten 10. At the instant when compression of air is completed by the piston 10, mixture gas is forced into the recess 10B and highly rich air-fuel mixture near an ignition plug 16 is ignited in a reliable manner. Then, combustion is propagated to lean mixture gas. With such an arrangement, it is enabled to stabilize combustion of mixture gas, to increase the engine output and to lower the fuel consumption.