An internal combustion engine includes a main combustion chamber and a small auxiliary combustion chamber. The main chamber is formed by a cylinder, the undersurface of a cylinder head, and the crown of a piston. A main ignition unit is disposed in the cylinder head, including a main body and a spark plug. The auxiliary chamber is formed within the main ignition unit. The main and auxiliary chambers are connected via a communication passage. The piston crown includes a cavity. When a fuel injection valve sprays fuel into the cavity on compression stroke, a tumble flow of fuel gas is generated to locally form a relatively rich air-fuel mixture near the communication passage. The air-fuel mixture partly flows into the auxiliary chamber as the piston travels upwardly. The air-fuel mixture in the auxiliary chamber is ignited by the spark plug so that a combustion flame propagates into the main chamber.