PURPOSE: To obtain easy and inexpensive disassembling and checking of a guide vane mechanism by furnishing a supporting member to support a servo-motor to be horizontally rotatable on a foundation member for facilitating the centering of the servo-motor.

CONSTITUTION: A support member 51 provided with a projection part 51b with a hole 51c on a fixing flange 51a is fixed on a foundation member 7 via an adjustment liner 52. A fixing arm 53 provided with an arm part 53b to be fitted in the intermediate gap part of the support member 51 on a fixing flange 53a and a hole 53c on the arm part 53b is fixed on a side flange 1d of a servo-motor 1, which is connected to the support member 51 by a pin 54 via the fixing arm 53. Meanwhile, a piston rod 1a of the servo-motor 1 is connected to the guide ring 2 by a pin 10 via a connection member 55 connected to the top screw part 1b. In this construction, the piston rod 1a of the servo-motor 1 is reciprocated to rotate the guide ring 2.