PURPOSE: To control the cooling water temperature on the outlet side at a constant level by using a pressure compensated flow control valve by changing the number of operating cooling fans based on the maximum opening or minimum opening of the pressure compensated flow control valve.

CONSTITUTION: When the flow rate of the cooling water becomes the state, at which the opening of the pressure compensated flow control valve 10 reaches the minimum opening VA, the number of the operating cooling fans 2W5 is increased by one. The temperature t2 of the cooling water flowing in a cooling water pipe 9 on the outlet side is quickly decreased from TA to TA'. At the same time, the opening of the pressure compensated flow control valve is moved from the minimum opening VA to VA' which is close to the maximum opening. As a result, the temperature t2 of the cooling water flowing in the cooling water pipe 9 on the outlet side, i.e. the cooling water temperature on the outlet side of the cooling tower, is controlled at the constant level, even though there is some fluctuation immediately after the increase of the number of the operating sets of the cooling fans 2W5.