PURPOSE: To prevent knocking in high compression ratio by providing a sub-chamber for cooling a portion of mixture gas in a position spaced farthest from an ignition source for combustible mixture gas filling a combustion chamber.

CONSTITUTION: Fuel in a combustion chamber 4 formed between the top surface 2a of piston and the inner wall surface 3a of cylinder head is ignited by an ignition plug 5 in the end of compression stroke to be propagated approximately in the form of sphere centering around an ignition plug electrode 5. As pressure in the combustion chamber 4 is raised, mixture gas spaced from the ignition plug 5 is cooled through a throttle portion 11 in a sub-chamber 10 so that it is prevented from self-ignition. On the other hand, a portion of mixture gas enters the sub-chamber 10 so that the mixture gas has neither so high pressure, self-ignition nor knocking in the combustion chamber 4.