PURPOSE: To eliminate the deflection of a cross beam with a reaction force being made parallel to the cross beam, by converting a clamp motion force in the horizontal direction along the longitudinal direction of the cross beam by a cylinder mechanism into a pressing force orthogonal with the support shaft of a clamp arm being slid.

CONSTITUTION: A cross beam 4 is made liftable along a column 2 between a pair of the right and left columns 2. A cylinder mechanism 5 is fixed in a horizontal posture in the longitudinal direction of the back side face both ends of the cross beam 4, the piston rod 5b thereof is longly projected horizontally in the column 2 direction. On the cross beam 4, a fixed support shaft 7 is provided at the position slid to the horizontal directional outside from on the piston rod 5n extension via a bracket at the column 2 side, and a clamp arm 8 turnable with the support shaft 7 as the fulcrum is provided. The tip of the piston rod 5b is pivotally attached by a pin 8a to the arm 8 one end, and a pressing member 9 is attached onto the abutment face of the arm 8 and column 2. Therefore the motion force in the horizontal direction of the rod 5a becomes the pressing force of a pressing member 9, and the deflection of the beam 4 is eliminated.