PURPOSE: To reduce the pressure loss in a gas cooler upon the partial load of an electric power generating plant and improve the efficiency of the electric power generating plant by a method wherein the opening and closing of opening and closing valves, connected respectively to a cooling water supplying pipeline connected to a cooling water tray, are effected in accordance with a turbine load signal.

CONSTITUTION: Informations how to open and close the opening and closing valves 15a, 15b, 15c in accordance with the turbine load signal S, are memorized in a control unit 17 previously and the control unit 17 opens and closes the valves 15a, 15b, 15c based on the informations. On the other hand, the open- ing and closing valve 15a is closed by the control unit 17 in accordance with the flow amount of gas upon the partial load of the electric power generating plant, the opening and closing valve 15b is opened and cooling water is supplied into the cooling water tray 12b. Accordingly, a heat exchange area, formed below the cooling water tray 12a arranged at the uppermost state, is eliminated in this case and, therefore, the pressure loss of the gas may be reduced.