An injection valve (21) is arranged in the vicinity of an inlet aperture (17) and is so formed that it injects fuel into the combustion chamber. A spark plug (22) is arranged more closely than the injection valve to the central part of the combustion chamber. A cavity (32) is so formed in the upper side of the piston that it is facing the injection valve and the spark plug. The shape of the cavity is such that a first curved surface (32b) under the spark plug and a second curved surface (32a) beneath the injection valve have first curvatures which are different from one another. The curvature of the first curved surface is greater than that of the second curved surface.
