PURPOSE: To improve air utilizing efficiency in a combustion chamber for diffusing combustion gas jetted from the nozzle hole of a vortex chamber to plural cavity parts through a diffusion accelerating part by setting appropriately the angle of the nozzle hole and the configuration and position of the diffusion accelerating part.

CONSTITUTION: When the points of both longitudinal edge extension lines 1f, 1r of a nozzle hole 6 starting to go over the tip part of a diffusion accelerating part 13 are respectively made a decay point and an end point in the down process of a piston 3, the angle θ of the nozzle hole 6 against a piston top face 8 is set in such a way that the crank angle of the decay point is within ATDC 17-21° and that of the end point within ATDC 33-35°. When the angle of a straight line PQ connecting a point P of the front edge extension line 1f of the nozzle hole 6 intersecting the bottom face of a cavity part in the position of a top dead point to the central points Q of bilateral large cavity parts 12, against the jet center line lo on the plane is made (α/2)°, the configuration of a tip outer shell part 13A in the diffusion accelerating part 3 is set along the straight line drawn from the point P toward the point Q, within the range of (α/2)±2°.