Abstract: For rotating the runner of a fluid machine in compressed air atmosphere in the runner chamber after shutting off working water and feeding compressed air to the runner chamber to move the liquid level therein downwardly below the runner while discharging water remaining in the outer peripheral portion of the runner through a leak-water exhaust pipe following starting of pump-operation, an inlet valve opening detector is mounted and operatively connected to a leak-water exhaust valve controller for controlling a leak-water exhaust valve. Stable operation of the fluid machine in spinning-in-air mode can be achieved by opening the leak-water exhaust valve by means of the leak-water exhaust valve controller following lapse of a given period of time after the inlet valve is fully closed, to permit the water remaining in the outer peripheral portion of the runner to be discharged through the leak-water exhaust pipe.