PURPOSE: To prevent the ring stick and secure the desired engine faculty by preventing the generation of high temperature at an abutment part formed by cutting a part in the peripheral direction of a piston ring.

CONSTITUTION: A piston 97 fitted into a cylinder hole 96 is constituted of a piston body 206 and a piston ring 215 which is externally fitted on the outer peripheral surface in the upper part of the piston body 206. A part in the peripheral direction of the piston ring 215 is cut, and the cut part is formed to an abutment part 216. Fuel 31 is jetted from one side in the radial direction of a combustion chamber 105 surrounded by the cylinder 95 and the piston 97 towards the other side. The turn of the piston ring 215 around the axis center of the piston body 206 is suppressed so that the abutment part 216 of the piston ring 215 is positioned on the other side of the combustion chamber 105.