Abstract: An internal combustion engine provided with a main combustion chamber, which is defined in the upper portion of a cylinder volume and has an intake valve and an exhaust valve, and a pre-combustion chamber free of an intake valve. In this internal combustion engine, the main combustion chamber is communicated by a passage with the pre-combustion chamber. The longitudinal axis of an ignition plug provided in the engine runs at a certain angle to that of said passage, the spark gap of the ignition plug being located within the passage thus permitting the combustion of a uniform, lean mixture and improving the ignitability thereof, while reducing the amount of harmful gas components contained in exhaust gas.