To suppress generation of white smoke and combustion noise by reducing the strength of a swirl in a swirl chamber during low speed operation.

An approximately spherical swirl chamber 6 having upper and lower parts the inner peripheral walls of which are formed in a semispherical is disposed at the upper part of one side of a main combustion chamber 5 between a piston 3 and a cylinder head 4 and a main injection nozzle 7 obliquely opened from the lower part of the swirl chamber 6 toward the central part of a main combustion chamber 5. An auxiliary injection nozzle 10 having a sectional area lower than that of the main injection nozzle 7 is formed from the side, situated closer to the central part of the main combustion chamber, of the swirl chamber 6 approximately in parallel to the central axis of the swirl chamber 6. A fuel injection nozzle 8 to inject fuel against the swirl chamber 6 is arranged. During low speed operation, a swirl is generated by air flowing in after the passage of it through the main injection nozzle 7 and meanwhile, air flows in a direction, extending opposite to a swirl, through the auxiliary injection nozzle 10 having a low sectional area and causes the occurrence of a disturbance to the swirl and reduces the swirl.