PURPOSE: To improve specific fuel consumption with smoke concentration lowered, by making up a cavity being offset at a certain distance from the center of a piston atop, while arranging a fuel jet nozzle at a fixed distance away from the center of that cavity.

CONSTITUTION: On the top of a piston 20, there is provided with a cavity 21 and the center of this cavity 21 is offset at a ceratin distance from a center D of a piston 20. A fuel jet nozzle 23 is further offset at a fixed distance away from the center of the cavity 21. The fuel jetted from 1st and 2nd nozzle holes 24 and 25 is apt to flow in 1st and 2nd sections where cubic contents are larger. On the other hand, fuel from the 3rd jet nozzle 26 is pointed to a section 30 at the downstream of swirls produced in a combustion chamber. Like this, smoke concentration in the exhaust can be lowered down and simultaneously improvement in specific fuel consumption can be much expected.