To generate an effective gas flow in the combustion chamber, and thereby enhance an air utilization rate by forming an upheaval part which is raised up from the bottom center caved in a recessed shape, in the apex surface of a piston, and thereby forming an inner wall surface partitioning a main combustion chamber while being faced to the outer side surface of the upheaval part roughly in parallel therewith.

A fuel injection valve 7 is set, which injects fuel in a radial shape in to a main combustion chamber 5 opened in the apex surface of a piston, and an upheaval part 15 in a conical shape is formed at the bottom center of the main combustion chamber 5. Namely, the upheaval part is so formed that its each cross section perpendicularly intersected with a cylinder center line C is circular, each cross section is so formed as to be gradually smaller as it goes up from below, and an inner wall surface 13 and the outer side surface 14 of the upheaval part 15 are formed into a conical surface shape where they are enlarged while an identical angle is being kept around the cylinder center line C. An inlet port part opened to the apex surface of the cylinder is formed into an inlet port surface 12 in a cylindrical surface shape, this constitution thereby eliminates an edge part formed by the apex surface 21 and the inner wall surface 13, and prevents the piston 4 from being excessively over loaded.