A combustion chamber (3) defined by a recess formed in the piston top (2) in the center axis (O₁) direction thereof with its geometric magnitude being progressively enlarged in the direction of the recess. The intensities of swirl flow (S) and squish flow (V) are optimized by letting the two flows (S,V) interact each other in the upstream side so as to optimize the air flow in the combustion chamber (3) overall. In the meantime, fuel is atomized and injected through a fuel injection nozzle means (15), which has five or more nozzle holes (5), toward the side wall (4a) of the combustion chamber (3) so as to circumferentially divide the chamber (3) by the fuel sprays (f) into five or more approximately equal sections. Thus, uniform dispersion and distribution of fuel together with the optimized flow of air is achieved in the combustion chamber (3), whereby the air utilization rate is raised and the combustion unburnt matters are decreased.