PURPOSE: To improve combustibility of lean mixture by producing turbulence in combustion chamber, through employment of an arrangement in which a pair of subsidiary combustion chambers are formed in combustion chamber of engine in the manner facing toward each other, so that combustion gas is issued symmetrically from two ports formed in respective subsidiary combustion chambers.

CONSTITUTION: A pair of subsidiary combustion chambers 10, 20 respectively having an ignition plug 23 are formed in a main combustion chamber 6 in the manner facing toward each other. Each of these subsidiary combustion chambers 10, 20 is formed with two large and small ports 11, 12 and 21, 22 opened in main combustion chamber 6, and these two ports are disposed symmetrically in pairs. With such an arrangement, combustion gas A, A' (or B, B') issued from large port 11 of one subsidiary combustion chamber 10 and small port 22 of the other subsidiary combustion chamber 20 collide with each other and swirl of combustion gas is developed in one direction, so that combustibility of mixture in main combustion chamber 6 can be improved.