PURPOSE: To stably stop a pump waterwheel by reducing the revolution speed of the pump waterwheel when a stop instruction is transmitted and by closing- controlling a guide vane and disconnecting the pump waterwheel from an electric power system when the opening degree reaches a prescribed opening degree.

CONSTITUTION: A Francis-type pump waterwheel is equipped with a plurality of flow-rate adjusting guide vanes 2 in the outer peripheral side flow passage of a runner 1, and each guide vane 2 is opening/closing-controlled by a guide vane controller 4 through an operating mechanism 3. A power generating motor 6 is directly connected to the runner 1 through a main shaft 5, and said motor 6 is controlled by a variable speed controller 7. When a stop instruction is generated, the revolution speed of the power generation motor 6, namely of the pump waterwheel is reduced to a prescribed revolution speed from a rated revolution speed N0 until a prescribed revolution speed reduction rate is realized, and the guide vane 2 is closing-controlled. Then, the pump waterwheel is disconnected from an electric power system, and the guide vane 2 is perfectly closed to stop the pump waterwheel.