In an oil jet system that directs an oil jet toward a back face of a piston, when a coolant temperature of an engine has increased to a predetermined temperature in a stopped state of the oil jet, the piston is cooled by starting the oil jet. When a lubricating oil temperature of the engine has decreased to a predetermined temperature in an operated state of the oil jet, a delay time is determined based on a variation amount in engine rotation speed and a variation amount in intake air filling factor during a predetermined period, and the oil jet is stopped after a lapse of the delay time.