PURPOSE: To reduce a discharge amount of NOx and smoke at partial load time by providing each two intake/exhaust valves around a fuel injection valve, and sucking residual gas in a cylinder once into an intake pipe by a negative pressure thereof, thereafter enabling the gas to again recirculate in the cylinder.

CONSTITUTION: Two intake ports 21 and two exhaust ports 22 are formed opposed to each other in a cylinder head 10, and each exhaust valve 9 and each intake valve 8, for opening/closing each port 21, 22, are opened with overlapping of about 40° by a crank angle. In the cylinder head 10, a fuel injection nozzle 6 is mounted to appear in a central part of a combustion chamber 5 further in parallel to a center axis of a cylinder 2. Two intake valve recesses 23, hollowed in a recessed shape below each intake valve 8, and two exhaust valve recesses 24, similarly hollowed in a recessed shape below each exhaust valve 9, are respectively formed around a cavity 4 in a top surface of a piston 3. On the halfway of an intake pipe, a throttle valve is interposed, and its opening is controlled in accordance with an operational condition.