To enable a shaft to be easily exchanged by providing a contact portion for contacting a deck portion side end part and a movable portion for displacing the contact portion axially according to spirally fitting to and spirally retracting from the contact portion.

A nut 29b spirally fitted to a shaft end part 29c is removed, a pin 41b is brought into contact with the shaft end part 29, and an exchange jig is set in a state in which a movable portion 42 is brought into contact with a frame 27 via a member 41d. If the movable 42 is spirally advanced in the direction of the member 41d, a U-letter shaped member 41a is displaced in the right direction, and the pin 41b is displaced to the right side. A shaft 29 is displaced in the reverse direction to a deck part 6 by pressurizing the end part 29c, the shaft 29a can be extracted from the right side. Since there may be only a space equivalent to the length of the pin 41b, even if an interval A between the deck part 6 and a driving device is narrow, the exchange jig can be set, and the shaft 29 can be easily extracted and exchanged by pressurizing the end shaft part 29c.