To prevent danger of water supply suspension due to troublesomeness of inspection/cleaning and clogging of a strainer of a water supply piping of a hydraulic turbine main shaft water sealing device by dispensing with it.

Clean water such as city water or well water is supplied to a water sealing device which seals between a main shaft 1 of a hydraulic turbine and a cover 2. A solenoid valve 13 of a water supply piping 4 and a water supply pump 14 are opened/closed and started/stopped synchronously to starting/ stopping and load interruption of the hydraulic turbine. Since the clean water is used, it is unnecessary to arrange a strainer. In addition, since the clean water is supplied at the time of starting/stopping or load interruption which require water supply from the outside, the consumption amount of the water is suppressed to minimum.