A cooling water temperature controlling device includes a heat exchanger for heat-exchanging between primary cooling water and secondary cooling water, a tank for storing the secondary cooling water, an injection pipe connecting the tank and a secondary cooling water supply unit, a valve for controlling a flow rate of the secondary cooling water from the secondary cooling water supply unit to the tank, a fluid level detection unit for detecting a level of the secondary cooling water in the tank, a first circulation pipe connecting the tank and the heat exchanger, a second circulation pipe connecting the heat exchanger and a temperature-control target, a third circulation pipe connecting the tank and the temperature-control target, a pump for circulating the secondary cooling water through the first to third circulation pipes, and a controller for controlling the valve and the pump based on the detected fluid level.
