PURPOSE: To provide an ignition system for an internal combustion engine with which the fuel is ignited by the heat of combustion gas.

CONSTITUTION: Besides a combustion chamber 2, the ignition system concerned includes an auxiliary combustion chamber 3 which is opened and closed by a valve 5 and is furnished with a fuel injection nozzle 4. Fuel is injected into a high temp., high pressure combustion gas in the auxiliary combustion chamber 3 in the condition closed by the valve and ignited with the heat of the combustion gas, and near the upper dead point of the piston 1 in the compression stroke, part of the high temp. and high pressure combustion gas and uncombusted portion of the fuel are jetted into the combustion chamber 2 from the auxiliary combustion chamber 3, and the fuel is distributed in the combustion chamber 2 and combusted with the kinetic energy which the jetted gas possesses. At the final stage of the combustion stroke, the auxiliary combustion chamber 3 is closed by the valve, and thereby part of the high temp., high pressure combustion gas is stored in the auxiliary combustion chamber 3. The initial ignition is made with an overheat plug.