PURPOSE: To promote the combustion so as to reduce the concentration of flue gas by generating moderate vortices in a swirl chamber either in a high speed range or a low speed range.

CONSTITUTION: An internal combustion engine has a main combustion chamber C defined by a cylinder, a cylinder head 26 and a piston 27, a swirl chamber 28 communicated with the combustion chamber C through two jet ports 30, 31 formed in the center part and the peripheral part of the combustion chamber C, and means (columns 32, 33) for changing the areas of the jet ports.