PURPOSE: To make possible a stable control of operation of cooling water system, by inputting detected values of ambient conditions, cooling water conditions, and operating state values of a plant into an automatic arithmetic unit, and by combining the number of operating fans of a cooling tower and blade angle of cooling water pumps (or the number of pumps) with each other.

CONSTITUTION: Provided on cooling-water lines for inlet and outlet of a condenser, are temperature transmitters 17, 18 to detect temperatures of cooling- water and discharge-pressure transmitters 19a, 19b to detect flow rate of cooling- water. Further, transmitters 20aW20e, 21a and 21b are provided for motors for cooling tower fans and cooling water pumps to detect number of those in operation. On the other hand, a transmitter 22 is provided to detect atmospheric temperatures and wet-bulb temperatures. An automatic arithmatic unit 24, into which signals from each transmitter are inputted, is provided to select economical means of operation from present operating conditions and optimum operating conditions so that the optimum operation of a cooling water system is attained.