To hold an auxiliary top plate in the same height as a main top plate by providing a fixing means for drawing out the auxiliary top plate fixed to the front end part of a vertically rotatable support frame from the main top plate and fixing it.

When an auxiliary top plate 2 is used, the auxiliary top plate 2 is drawn forward from a main top plate 1. The engaging pin 7 of a bent part 6 is slid forward along a guide slit 9 by this drawing, and the auxiliary top plate 2 is then lifted up to conform the rear end surface to the front end part of the main top plate 1. The flat part 4 of a support frame 3 makes contact with the lower surface of the main top plate 1, whereby the upper surface of the auxiliary top plate 2 is flushed with the main top plate 1. In this state, the through-hole 12 of the support frame 3 accords to the through-hole of the main top plate 1, and a fixing means 10 is rotated, whereby its screw part 10a can be screwed to the screw hole of the main top plate 1. The auxiliary top plate 2 can be fixed flush with the main top plate 1 by this screwing, and the auxiliary top plate 2 can be held in a convenient state.