PURPOSE: To improve cooling performance of a top ring and a to land by forming a ring-shaped cooling ring to which an oil is circulated coaxially with the piston center axis on the back of a piston ring groove and by inserting a ring-shaped heat conductor to this cooling ring so that one end is faced with it.

CONSTITUTION: In a diesel engine provided with an oil jet type piston, a ring- shaped cooling ring 4 is formed coaxially with the piston center axis on the back of a piston ring groove 10, and oil is flowed from a in-flow port 6 for receiving an oil jet 3 blown out of a nozzle 2 to an out-flow port 7. At the upper end of this cooling ring 4, a heat conductor 5 in the ring shaped whose other end is extended close to the outer circumferential face of the piston top face is inserted into the body. This heat conductor 5 is formed by a material with the heat conductivity higher than that of the body and arranged coaxially with the piston center axis. When a cooling oil is flowed, the heat conductor 5 plays the role of a cooling pipe to obtain an excellent cooling performance.