Disclosed is an oil jet device for cooling a piston, including: an oil cooler 4 that is disposed upstream of an oil injection nozzle 8; an oil pump 5 that is disposed upstream of the oil cooler 4; a first switching adjustment valve 6 that is disposed between the oil injection nozzle 8 and the oil cooler 4 and adjusts a flow dividing ratio at which the cooling oil from the oil cooler 4 is distributed to the oil injection nozzle 8 side and to an oil pan side; and a control unit that has an oil quantity adjustment map 4 for switching the first switching adjustment valve 6 based on a piston temperature calculation map 20 for calculating the temperature of the piston 1 using detection values acquired respectively by a cooling water temperature sensor 35, a rotation speed sensor 36 and a load sensor 37.