PURPOSE: To protect a piston, cylinder walls, and the like from overheat by jetting lubricating oil, discharged from an oil pump, toward the back face of the piston from oil jet nozzles provided cylinder by cylinder, through a nozzle through hole.

CONSTITUTION: At the time of low rotation of an engine where the discharge pressure of an oil pump 1 is lower than the specified value, a hydraulic valve 15 is closed, and the jet of lubricating oil from each oil jet nozzle 10 is stopped. At this time, the lubricating oil discharged from the pump 1 is partially fed to each main bearing 8 from a main gallery 6, and the remaining lubricating oil is fed to a valve system 9 from a head gallery 7. When the pump discharge pressure rises exceeding the specified value at the time of high rotation, a valve body moves against a return spring, and the valve 15 is opened. Part of lubricating oil is thereby fed to the oil jet nozzles 10 through an oil jet gallery 11, and the lubricating oil jetted from the oil jet nozzles 10 comes in contact with the back face of a piston to take away heat so as to prevent the overheat of the piston, cylinders, and the like.