At an inner circumference side of a lower ring and an upper ring, flange pars are formed respectively, and a ring-shaped clamp device is arranged at the inner circumference side, and separated at four points, into a front side, back side, left side, and right side, and divided into four divided clamps. At the separation points of front and back sides, the left and right divided clamps are rotatably connected with each other, and at the separation points of left and right sides, the front and back divided clamps are connected to cylinders respectively. By the extension/contraction operation, the front and back clamps move between joint and separation positions, and if the front and back clamps position at the joint points, predetermined spaces are formed between the divided clamps. Lock blocks to be inserted into the spaces for regulating return of the divided clamps to the separation positions are provided.