The cooling water temperature control device includes a thermo valve 2 that opens a main flow passage R1 when the temperature of a sensed region R1a in the main flow passage R1 reaches or exceeds an operating temperature set in advance, a sub-valve 3 that opens and closes a sub-flow passage R2 bypassing the thermo valve 2, and a control unit 6 that opens the sub-valve 3 when the temperature of the cooling water on an upstream side of the thermo valve 2 is a predetermined temperature. One end on the upstream side of the sub-flow passage R2 is connected to the sensed region R1a in the main flow passage R1 or to a downstream side of the sensed region R1a, and the operating temperature of the thermo valve 2 is set to be equal to or lower than the predetermined temperature.