PURPOSE: To improve the cooling effect at the piston ring section by setting a specific relation between the diameters of an annular cooling path provided around the axis of piston in the piston body and the inlet/outlet paths communicating respectively with said cooling path.

CONSTITUTION: The diameter D3 at the inlet of piston cooling oil inlet path 2 is set larger than the diameter D4 of said path and provided below a piston skirt 9. D3 is made larger than the outer diameter D1 of a piston cooling oil ejection nozzle 7 while the diameter D2 at the ejection end of said nozzle 7 is made approximately same or smaller than the outer diameter D1. Consequently, the spray ejected through said nozzle 7 can be fed reliably through said path 2 into an annular cooling path 3 to cool the piston ring 5 section thus to lower the temperature of the piston crown 1 and the piston skirt 9, resulting in prevention of burning of the stick of piston ring 5 or the piston skirt 9.