To provide a cooling water temperature control device capable of regulating cooling water temperature while suppressing efficiency drop of accessory for a vehicle.

The cooling water temperature control device is provided with a sub engine 20 driving an alternator 36, a temperature raise demand detection part 60A detecting cooling water temperature raise demand based on an operation condition of a heating device 1, and a control part 60B increasing quantity of fuel supplied to the sub engine 20 based on cooling water temperature T and regulating electric power quantity supplied to PTC heater 92 consuming electric power generated by the alternator 36 based on increased fuel quantity when cooling water temperature raise demand is detected by the temperature raise demand detection part 60A.