To efficiently attach and fix a bed part while it is advantageous in terms of manufacture and a backlash is less generated.

For a bed part attaching structure, a frame 26 constituting the bed part is provided with an L-shaped locking member 22 composed of a vertical part 22B projected downwards from its bottom surface and a locking part 22C extended from the lower end of the vertical part 22B in a horizontal direction. At the attaching part of a bed main body, an opening 24 to which the locking part 22C of the locking member 22 can be inserted from above is formed. The frame 26 of the bed part is provided with a regulation member 25 for regulating the movement to an unlocking side of the locking part 22C by entering a gap generated between the end part on the opposite side of the locking side end part of the locking part 22C and the edge part of the opening 24 at a locking position at which the tip part of the locking part 22C inserted to the opening 24 from above is moved to a front part and the locking part 22C is prevented from being detached from the opening 24 to the upper side.