A cooling device for an engine including an oil jet device for cooling a piston with oil, the cooling device comprising: a cooling water temperature sensor that is designed to detect a temperature of the engine; a rotation speed sensor that is designed to detect rotation speed of the engine; a load sensor that is designed to detect load of the engine; a jet nozzle that is designed to be secured in a cylinder block of the engine and is configured to inject cooling oil onto the rear face of the piston; an oil cooler disposed upstream of the jet nozzle on a distribution path (11; 12) of the cooling oil; an oil pump that is located upstream of the oil cooler and is configured to pump the cooling oil to the oil cooler; a first switching adjustment valve that is disposed between the jet nozzle and the oil cooler, and is configured to adjust a flow dividing ratio at which the cooling oil from the oil cooler is distributed to the jet nozzle side and to an oil pan side; a control unit (30; 40; 50) having an oil quantity adjustment map for determining an opening degree of the first switching adjustment valve based on a piston temperature calculation map for calculating the temperature of the piston using the detection values acquired respectively by the temperature sensor, the rotation speed sensor and the load sensor; the cooling device being characterized in that it further comprises: a second switching adjustment valve disposed between the oil cooler and the oil pump on the distribution path of the cooling oil, and configured to adjust a flow dividing ratio at which the cooling oil from the oil pump is distributed to the oil cooler side and to a bypass circuit side, the bypass circuit being connected between the oil cooler and the first switching adjustment valve, wherein the control unit (40; 50) has an oil temperature adjustment map for determining opening degree of the second switching adjustment valve based on the piston temperature, whereby the temperature of the cooling oil, after passing through the bypass circuit, is adjusted. a cooling water temperature sensor that is designed to detect a temperature of the engine; a rotation speed sensor that is designed to detect rotation speed of the engine; a load sensor that is designed to detect load of the engine; a jet nozzle that is designed to be secured in a cylinder block of the engine and is configured to inject cooling oil onto the rear face of the piston; an oil cooler disposed upstream of the jet nozzle on a distribution path (11; 12) of the cooling oil; an oil pump that is located upstream of the oil cooler and is configured to pump the cooling oil to the oil cooler; a first switching adjustment valve that is disposed between the jet nozzle and the oil cooler, and is configured to adjust a flow dividing ratio at which the cooling oil from the oil cooler is distributed to the jet nozzle side and to an oil pan side; a control unit (30; 40; 50) having an oil quantity adjustment map for determining an opening degree of the first switching adjustment valve based on a piston temperature calculation map for calculating the temperature of the piston using the detection values acquired respectively by the temperature sensor, the rotation speed sensor and the load sensor; the cooling device being characterized in that it further comprises: a second switching adjustment valve disposed between the oil cooler and the oil pump on the distribution path of the cooling oil, and configured to adjust a flow dividing ratio at which the cooling oil from the oil pump is distributed to the oil cooler side and to a bypass circuit side, the bypass circuit being connected between the oil cooler and the first switching adjustment valve, wherein the control unit (40; 50) has an oil temperature adjustment map for determining opening degree of the second switching adjustment valve based on the piston temperature, whereby the temperature of the cooling oil, after passing through the bypass circuit, is adjusted. a second switching adjustment valve disposed between the oil cooler and the oil pump on the distribution path of the cooling oil, and configured to adjust a flow dividing ratio at which the cooling oil from the oil pump is distributed to the oil cooler side and to a bypass circuit side, the bypass circuit being connected between the oil cooler and the first switching adjustment valve, wherein the control unit (40; 50) has an oil temperature adjustment map for determining opening degree of the second switching adjustment valve based on the piston temperature, whereby the temperature of the cooling oil, after passing through the bypass circuit, is adjusted.