PURPOSE: To prevent the occurrence of water hammer in a position in the vicinity of a cendenser back washing device through throttling of an amount of cooling water for a condenser, by a method wherein, by means of a switching starting signal from the back washing valve of a condenser, the opening of a valve on the downstream side of the back washing valve of the condenser is regulated to a specified value to throttle an amount of cooling water for the condenser.,

CONSTITUTION: By means of a switching starting signal 8 from a back washing valve change- over switch 7, the opening of an outlet valve 4a of a back washing valve is throttled through an opening regulating device 9 of an outlet valve 4 of a back washing device. After elapse of a given time, the opening of the outlet valve 4 of the back washing valve is restored to a value before regulation through the working of an opening regulating device 9 for the outlet valve 4 of the back washing valve. By throttling the opening of the outlet valve of the back washing valve during switching of the back washing valve, an amount of cooling water flowing through a cooling water system 6 is decreased to decrease an increase in a pressure due to water hammer phenomenon around a back washing valve 3. This decrease enables the use of a back washing valve, being switched in a short time, and achieves simultaneousIy two purposes of the one being prevention of lowering of the degree of vacuum of the condenser and the other being the prevention of the occurrence of water hammer phenomenon.