A combustion chamber system of a kerosine internal combustion engine including a main combustion chamber and an auxiliary combustion chamber both of which are defined in a cylinder head and are spaced from each other by a partition. A plurality of communication holes through which these main and auxiliary combustion chambers communicate with each other are made in the partition. The main combustion chamber has a suction port and an exhaust port opened thereto which are adapted to be opened and closed by a suction valve and an exhaust valve, respectively. The open ends of the communication holes on the side of the main combustion chamber are pointed to the upper surface of a piston located in the vicinity of a top dead center thereof, and a mixture of a theoretical or richer fuel ratio is fed into the main combustion chamber.