PURPOSE: To prevent generation of smoke accompanying deterioration of combustion caused by timing retard for reducing especially the amount of nitrogen oxides in a diesel engine.

CONSTITUTION: The inner circumferential surface of a combustion chamber 21 which consists of a recessed part formed on the top part of a piston 12 is formed in such constitution that an inclined collision surface 71 on which fuel atomization 70 collide diagonally, and a curved wall surface 72 formed adjacently to the inclined collision surface 71 in direction where the angle of the collision surface 71 to injection direction is large, are formed on respective injection ports.