PURPOSE: To stabilize combustion and suppress an amount of discharging NOX even when an air-fuel ratio is brought into a lean state.

CONSTITUTION: An intake control valve 16 the bottom wall of which is moved to the top wall side is located in an air intake passage. A bypass passage 19 to cause the flow of air-fuel mixture around the intake control valve to a spot situated closer to an ignition plug 5 is provided. In such a state that the intake control valve is closed, a tumble occurs. A layer formed of rich air-fuel mixture is formed in the vicinity of an ignition plug. Even when an air-fuel ratio is brought into a lean state, stable ignition is carried out and combustion is stabilized. Comparatively rich air-fuel mixture is burnt in the initial stage state of combustion, extremely lean air-fuel mixture is burnt at the terminal stage of combustion and an amount of generating NOX is reduced.