PURPOSE: To improve the irritating odor of exhaust gas and improve the maximum output, exhaust gas color, and fuel consumption by forming a recess in a windmill shape and forming a slant face on the upper portion from the middle of an intermediate wall face in an internal-combustion engine having a recessed main combustion chamber at the top of a piston.

CONSTITUTION: A main combustion chamber made with a recess 2 is provided at the top of a piston 1. A distance L1 from an injection port center N0 to the wall face 2A hit by fuel is set in relation with a cylinder inter-diameter D so as to satisfy an equation L1=(0.25W0.35)×D, and a number of faces 2A hit by fuel are formed in response to individual injection ports. An intermediate wall face 2B on the swirl outflow side is set at a distance L2 0.8W0.9 times distance L1 and has the curvature γ2 larger than the curvature γ1. The recess 2 is formed in a windmill shape by connecting the wall face 2A and intermediate wall face 2B. A slant face 4 having a slant angle of 5°W15° is formed on the upper portion from the middle of the intermediate wall face 2B. Accordingly, the irritating odor of exhaust gas can be improved and the maximum output, exhaust gas color, and fuel consumption can be also improved.