PURPOSE: To introduce intake air in a main chamber to the upper part of an auxiliary chamber and to inject ignited air-fuel mixture to the lower part of the auxiliary chamber to the main chamber.

CONSTITUTION: A fuel injection nozzle 16 is disposed to the upper part of an auxiliary chamber 18 arranged at the central part of a cylinder 26 and a conical valve seat 24 is formed at the lower part of a valve hole 19 through which the auxiliary chamber 18 and a main chamber 28 are intercoupled. A conical part 25 adjacent to a the valve seat 24 is formed at a spool 20 inserted in the valve hole 19. A blow down passage 23 extending from the upper part of the peripheral wall of the valve hole 19 and communicated with the main chamber 28 is provided. A check valve 12 to allow a flow from the main chamber 28 to the auxiliary chamber 18 is located in the middle of an inflow passage 14 through which the lower part of the the peripheral wall of the valve hole 19 is coupled to the upper part of the auxiliary chamber 18. Trough vertical movement of a spool 20 related to the stroke of an engine, the inflow passage 14 and the blow-down passage 23 are alternately opened and closed.