PURPOSE: To reduce a generated amount of NOx by flatting a piston head, and forming a main combustion chamber between this piston head and an opposed surface thereto of a cylinder head, so that a combustion time is lengthened to decrease the highest combustion temperature because of slowing fluidization of a mixture in the main combustion chamber.

CONSTITUTION: A dome-shaped subcombustion chamber 3 is hollowly provided in a piston head-opposed surface 2 of a cylinder head 1, and an opening part 4 of this subsidiary combustion chamber 3 appears in a main combustion chamber 5. A piston head 7, in which a spark plug 7 appears in the deepest end of the subsidiary combustion chamber 3, is flatted, and the main combustion chamber 5 is formed between these piston head 7 and piston head-opposed surface 2 of the cylinder head 1. In a compression stroke, since a mixture in a cylinder 8 is uniformly compressed by the flat piston head 7, a mixture is slowly fluidized in the main combustion chamber 5, and since the mixture is slowly mixed with unburnt gas from the subsidiary combustion chamber 3 to lengthen a combustion time in the main combustion chamber 5, a maximum combustion temperature is decreased to reduce the NOx.