Abstract: A diesel engine having an auxiliary combustion chamber which includes a cylindrical auxiliary combustion chamber, which is vertically formed in the center part of the cylinder head so as to be able to communicate with a main combustion chamber; a fuel injection device, mounted above the auxiliary combustion chamber of the cylinder head and which injects fuel into the auxiliary combustion chamber; and a piston, having a protrusion part, protruding to a fixed height in the center and upper part of the piston and of slightly tapered shape so that a communicating hole of the auxiliary combustion chamber, which communicates with the main combustion chasmber, can be opened and closed.