PURPOSE: To simplify the constitution by applying oil pressure and the like to one point of force and thus enabling the simultaneous clamping of each clamping mechanism to each strut.

CONSTITUTION: When oil pressure is supplied to the port 19 of an oil-hydraulic jack 15, a plunger 16 is lowered. The oil pressure acts, as external force, upon the plunger 16 and the point of force 21 of each of links 17 and 18 perpendicularly to the clamping directions of the clamp shoes 13 and 14 of clamping mechanisms 11 and 12. At this time, component forces in the lateral directions of the exterior force acting upon the point of force 21 are given to clamp shoes 13 and 14 respectively by the links 17 and 18. By this, the clamp shoes 13 and 14 are pressed outward against struts 9 and 10, being clamped simultaneously to the struts 9 and 10.