To provide a diesel engine, using low quality fuel, to improve fluidity of air and fuel by injecting low quality fuel, such as heavy oil, in a cylinder at the initial stage of a compression stroke and suppress the generation of NOx by generating uniform air-fuel mixture.

In a diesel engine, an auxiliary combustion chamber 2 is formed in a piston 5 and a main combustion chamber 1 consists of a combustion chamber structure 3 arranged in the cavity 9 of a cylinder head 7. A plurality of communication holes 15 radially formed and a fuel injection hole 13 in the shape of a flux are formed in the piston 5. Low quality fuel, such as heavy oil, is injected in a cylinder 8 through a fuel injection nozzle 6 at the initial stage of a compression stroke. The fuel is dispersed in a swirl present in the cylinder 8, uniform air-fuel mixture of fuel and air is generated in the cylinder 8, and fuel for ignition is reliably fed through the fuel injection nozzle 13 in the auxiliary combustion chamber 2 at the terminal stage of the compression stroke and ignited for combustion in the auxiliary combustion chamber 2.