PURPOSE: To stabilize combustion over the whole area of an engine operating area even if an air-fuel ratio is made lean.

CONSTITUTION: An intake air control valve 22 to move a bottom wall of an intake air passage B to the top wall side is arranged in a cylinder head 3. Bypass passages 24 and 29 and a fuel injection hole 30 to inject fuel by crossing these are formed in a top wall. A tumble is caused by the intake air control valve 22 at low load operation time, and an air-fuel mixture denser in a layer shape is made to flow around a spark plug 6. Therefore, ignition at the low load operation time is stabilized, and combustion can be stabilized over the whole area of an engine operating area.