PURPOSE: To prevent staying of air to discharge water in a runner chamber perfectly and reduce necessary electric power by a method wherein an air venting valve is provided at the upper part of the inlet port of a scroll chamber and the air venting valve is opened during the phase modifying operation to discharge leak air in the scroll chamber condituously.

CONSTITUTION: The automatic air vent valve 9 is provided at the upper part 8' of the scroll chamber 8, in which the air is ready to stay. When the command of the phase modifying operation is outputted, a guide vane 1 is closed gradually to the totally closed condition thereof, next, an inlet port valve 10 and a bypass valve 11 are closed totally sequentially. A water supplementing valve 12 is closed and pushing down of the runner chamber 3 is effected by using feed water for runner cooling water and an air feeding valve 2. When the pushing down is finished, the air feeding valve 2 is closed and a water discharging valve 5 is opened to discharge staying water in a priming part. Subsequently, the automatic air venting valve 9 is opened and leak air, entering into the scroll chamber 8, is discharged continuously. According to this method, staying of air may be prevented, the electric power for the idling of a runner may be reduced and the necessary electric power may be reduced.