PROBLEM TO BE SOLVED: To solve the resonance phenomenon of a common shaft by providing annular recessed grooves in several positions of the outer wall of the common shaft to install O-rings, and bringing a main shaft into contact with the common shaft in a part of the clearance formed between the inner wall of the main shaft and the outer wall of the common shaft through the O-rings. SOLUTION: Annular recessed grooves 11 are provided in several positions of the outer wall 8a of a common shaft 8, and O-rings 12 are installed to the recessed grooves 11. A main shaft 7 is brought into contact with the common shaft 8 in a part of the clearance S between the inner wall 7a of the main shaft 7 and the outer wall 8a of the common shaft 8 through the O-rings 12. The recessed grooves 11 are set to a dimension such that the circumferences of the O-rings 12 make contact with the inner wall 7a of the main shaft 7 when the 0-rings 12 are installed. Since the common shaft 8 is supported by the main shaft 7 in the contact positions, vibration of the common shaft 8 is surely suppressed. Thus, the resonance phenomenon of the common shaft can be prevented with a simple and inexpensive structure.