PURPOSE: To enable the temperature of cooling water produced by a water cooling tower to be maintained at a fixed target temperature, by a method wherein the temperature and the quantity of hot water returned to a water cooling tower are detected, a residual heat quantity is calculated from the detected values, and a cooling capability according to the residual heat quantity is calculated.

CONSTITUTION: The first temperature sensor Th1 for detecting the outlet temperature T1 of cold water fed out from the water cooling tower 1, the first flow rate sensor FQ1 for detecting the flow rate Q1 per unit time of cold water and a pressure sensor FP for detecting the pressure P of cold water are intermediately provided in a main forward passage 2. The second and third temperature sensors Th2, Th3 for detecting the inlet temperatures of hot water returned from a device 3 to be cooled and the second and third flow rate sensors FQ2, FQ3 for detecting the flow rates Q2, Q3 per unit time of returned hot water are provided respectively in a branch backward passage 6a and a confluent passage 6bc of passages 6b, 6c. Each of the values detected by the sensors is inputted into a controller 7, and feed-forward control is conducted so as to display a cooling capability according to the residual heat quantity.