PURPOSE: To prevent occurrence of an accident such as the damage of guide ring bearings, by employing such an arrangement that hydraulic pressure for opening guide vanes cannot be acted under the state that the guide vanes are locked in their closed position by a mechanical locking means.

CONSTITUTION: When a lock piece 12 is inserted between stoppers 9-1 and 10-3 in the state that guide vanes 1 are closed, the electric circuit of a changeover switch 16 is closed, so that a solenoid 17 is energized. Resultantly, hydraulic pressure 19-1 is acted to a pilot piston 14-1 via a line 19-1 and raises the piston 14-1 upward, so that hydraulic pressure 22 is acted to a piston 10-2 via a line 21-1 to depress a servo rod 9 downward. Therefore, the guide vanes 1 are kept in their closed state and hydraulic pressure 22-1 for opening the guide vanes 1 is prevented from acting to the piston 10-2. With such an arrangement, it is enabled to prevent damage of the guide ring bearings 7 caused by the hydraulic pressure for opening the guide vanes 1.