PURPOSE: To provide an engine which hardly causes a knocking even when the compression ratio is increased and the ignition timing is advanced to improve the heat efficiency.

CONSTITUTION: A sub-piston 19 is supported on the upper surface of a main piston 4, a mixture cooling chamber 20 is formed between both the pistons 4, 19, and the mixture cooling chamber 20 is allowed to communicate with the peripheral part of a combustion chamber 12. The sub-piston 19 is connected to a cam member 23 supported on a crankshaft 7 through a sub-connecting rod 21, and the capacity of the mixture cooling chamber 20 is extended and contracted, interlocking with the rotation of the crankshaft. The capacity of the mixture cooling chamber 20 is extended from the compressing stroke to just after ignition to cool the fuel-air mixture filled therein, whereby a knocking is prevented. The capacity is contracted just after ignition, and the fuel-air mixture in the mixture cooling chamber 20 is pushed to the combustion chamber 20 and burnt.