A device for controlling the temperature of cooling water includes a three-way valve having a first inlet, a second inlet, and an outlet; a first feed pipe; a second feed pipe; and a return pipe for connecting between an outlet of the temperature-control target and an inlet of the cooling water supply unit. The device also includes a return-side bypass pipe for connecting between the return pipe and the second inlet of the three-way valve; a pump provided on the second feed pipe for circulating the cooling water between the three-way valve and the temperature-control target; and a temperature measuring unit for measuring a temperature of the cooling water flowing in the second feed pipe. In addition, the device includes a controller for controlling the three-way valve and the pump in accordance with a detection result of the temperature measuring unit.