To supply water to a seal water device preferentially for cooling water for a power generator stator, a bearing part, etc., and to avoid an abnormal state such as a power generator stop even if a flow rate decreases owing to rubbish etc., during water discharge.

A water supply flow controller includes: three A, B, and C electric valves 451, 452, and 453 which are connected to a water supply system and provided for purposes of cooling of a power generator, cooling of a bearing part of a waterwheel, and water sealing of the bearing part so as to control a flow rate; a setting part 18 which sets priority levels associated with flow rate distributions by the purposes adjusted by the A to C electric valves; a plurality of flowmeters 46-48 which are connected to the water supply system, and detect water supply flow rates for the purposes whose priority levels are set by the setting part 18; and a control section 53 which is connected to the A-C electric valves, setting part 18, and flowmeters 46-48, and controls the A to D electric valves based upon the detected water supply flow rates for the purposes and the set priority levels.