PURPOSE: To suppress water hammer caused by the flow by performing the opening control of guide vane corresponding to the variation of rotary speed.

CONSTITUTION: A rotary speed control command is transmitted to a movable guide vane 2 at the low pressure stage to perform the opening control corresponding to the rotary speed variation. An inlet valve 7 is controlled independently such that it is held at predetermined opening smaller than the opening area of the movable guide vane 2 not to induce the flow variation or that the existing flow variation is suppressed positively on the basis of control command different from the control command of rotary speed. Consequently, variation of hydraulic pressure is prevented while the rotary speed control by means of the guide vane 2 is performed under stabilized condition of hydraulic pressure resulting in quick synchronization to the specific rotary speed.