PURPOSE: To improve ignition and operate the engine stably by a method wherein a main combustion chamber, formed by recesses provided on the inner side of a cylinder head and on the upper surface of a piston, is constructed to have a specified configuration.

CONSTITUTION: A precombustion chamber 64 is formed by the first recess 56 of the cylinder head 54 and the upper surface of the piston 44. The main combustion chamber 66 is formed by the second recess 58 of the cylinder head 54 and the recess 62 of the piston 44 so that it is wide at the side of the precombustion chamber 64 and is narrowed gradually when it approaches to the side of the exhaust port 52. At the initial frame propagating position for the main combustion chamber 66, the area of the flame is large and the most of mixture gas may be burnt. According to this method, a good ignition property may be kept in a low-speed low-load operating zone of the engine while a stable operating condition may be obtained in the high-speed high-load operating zone of the engine.