PURPOSE: To prevent the recessed position from crack thus to maintain durability by forming a recess in the circumferential wall face of a cavity on the top of a piston of the combustion chamber of a direct injection Diesel engine.

CONSTITUTION: A combustion chamber or a cavity 2 is provided in the top face of a piston 1. A pocket recess 4 is formed in the circumferential wall face of a cavity 2 formed between axis X of the piston 3 and axis Y crossing perpendicularly with axis X at the center of the plane of the cavity 2. In other words, said recess 4 is formed at the position keeping away from axis X where the tension is high and axis Y where the compression stress is high. Consequently even when the piston 1 has deformed, the recess forming position is prevented from crack to maintain durability.