A combustion chamber (20) opening toward a cylinder head (5) is provided on a top surface (11) of a piston (10), and this combustion chamber (20) comprises a first volume (22) having an inclined surface (20) and a second volume (23) further recessed from the first volume (22) toward a pin boss (13). Fuel spray F from a fuel injection nozzle (9) is injected toward an inner peripheral wall section (24) of the second volume (23) in a former stage of fuel injection and toward the inclined surface (21) of the first volume (22) in a later stage of fuel injection, and the percentage of the fuel injection period in the former stage against the total fuel injection period is set to the range from 40% to 70%.