PURPOSE: To perform reliable combustion of lean air-fuel mixture fed in a main combustion chamber by a method wherein a plurality of valve functions are provided together, an auxiliary combustion chamber is formed in a valve, the opening of a through-hole is regulatable, the volume of the auxiliary combustion chamber is changeable, and a valve hole varying timing is regulatable.

CONSTITUTION: A composite cylinder 1 simultaneously regulates a plurality of auxiliary intake air passages B and B' and a through-hole C, and further an ignition plug 3 is incorporated in a member 2 in a valve cylinder for integral formation. In movement of the member 2 in a valve cylinder, a chamber volume is increased during low speed rotation and a chamber volume is reduced during high speed rotation. In circular movement, when rotational movement is carried out, release through auxiliary intake air passages B and B' is effected, in order, from lean air-fuel mixture to rich air-fuel mixture. When reverse movement is carried out, release is effected in the order of rich, lean, and rich. In driving of a composite valve cylinder 1, a power from a cam shaft is brought into circular movement, and the member 2 in a valve cylinder is moved through the force of a spring 9 and by means of an oil pressure. This constitution causes reliable combustion of lean air-fuel mixture.