In a direct fuel injection diesel engine equipped with a pentroof-shaped piston, when fuel is injected into a cavity (25) recessed in a central part of a piston (13), for which the height of a top face changes in the circumferential direction, from a fuel injection point (Oinj) of a fuel injector disposed on a piston central axis along a plurality of fuel injection axes (Li1,Li2), if a cross-section of the cavity (25) passing along the fuel injection axis (Li1,Li2) is defined as a fuel injection cross-section (Sn), a cross-sectional shape (see shaded portion) of the cavity (25) defined by first to third specific points (An, Bn, Cn) on the fuel injection cross-section (Sn) is set so as to be substantially equal for each fuel injection cross-section (Sn). By so doing, the conditions in which fuel and air are mixed in each fuel injection cross-section (Sn) can be made uniform, the engine output can be improved, and harmful exhaust substances can be reduced.