PURPOSE: To suppress the increase of the temperature of a piston by preventing the occurrence of blow-bye of unburnt fuel and performing excellent cooling of the piston by means of fuel.

CONSTITUTION: A fuel injection device 7 is installed to a cylinder head 2 placed on a cylinder block 1 and a piston 4 is reciprocatively movably contained in a cylinder block. A combustion chamber 5 is formed in a manner to be surrounded with a piston and a cylinder head. A bowl part 11 is formed at the head part of the piston and fuel 10 is injected through a fuel injection device toward the bowl bottom 12 of the bowl part. The injected fuel is injected in a manner to make contact with a bowl bottom in a region wherein a radius centering around the center 0 of the bowl bottom is 1/4 of a diameter D2 of the mouth of the bowl part.