An internal combustion engine comprises a cylinder block and a cylinder head. A combustion chamber is formed in the cylinder block and the cylinder head. An accumulation chamber is formed within the cylinder head and communicated with the combustion chamber via an operating valve. The accumulation chamber is disposed adjacent to an exhaust port of the engine and is heat exchangeable with the exhaust port. The operating valve is opened approximately at the time the compression stroke is commenced and is closed approximately at the time the compression stroke is completed. A cap is disposed at the entrance of the accumulation chamber and has a plurality of jets formed thereon. The compressed combustible gas mixture is accumulated within the accumulation chamber, heated by the heat transmitted from the exhaust port and spouted out into the combustion chamber, so that approximately uniform turbulences are generated in the combustion chamber and the flame speed is increased.