PURPOSE: To achieve a rapid perfect combustion of fuel by a method wherein an auxiliary piston provided for a piston is loosely fitted in an opening of an auxiliary combustion chamber provided at the cylinder head, and a pressure difference is established between the main combustion chamber and the auxiliary combustion chamber so as to generate a swirl.

CONSTITUTION: A piston 5 is fitted in a cylinder 4, and an auxiliary piston 5a provided for the piston 5 gives a throttle effect at the clearance between itself and an auxiliary combustion chamber 6a provided at the cylinder head 6, thereby generating a swirl. Perfect combustion of fuel is effected by igniting the fuel in the swirl.