To prevent useless consumption of water due to overflow immediately after stoppage of operation while securing necessary water quantity in starting operation by changing set water levels for replenishing control between operation time in day and non-operation time at night.

At non-operation time when actuation of a pump 5 to forcedly feed water to a water work 4 from a supply water tank 1 is stopped, an operating valve 2 such as a solenoid valve of an automatic water replenishing system 3 is controlled to be opened/closed to constantly keep a water level in the supply water tank 1 at an initial water level IWL by receiving a first set water level detection signal of a water level detector 7, and at operation time when the pump 5 is actuated, the opening/closing valve of the automatic water replenishing system 3 is controlled to be opened/closed to constantly keep the water level in the supply water tank 1 at an operation water level DWL lower than the initial water level IWL for water quantity fed to a circulation water conduit 6 by receiving a second set water level detection signal of the water level detector 7.