To shorten work time for attaching/detaching a nut.

A pair of half nuts 4 integrally constituted of a half female screw 4 of an outer surface tapered body and a guide key 5 are positioned in a holder 2 and assembled on a base 1 (having a guide function) with a straight key groove passing through the center of the product. On the other hand, an inner surface of a set upper 3 is brought into contact with an outer surface tapered portion of the half nuts 4 positioned on the opposite of the attached base. The motion of the half nuts is converted into the orthogonal direction (a direction for clamping the bolt) by insertion of the set upper 3, and the female screws are moved toward inner/outer periphery direction to control "to engage" and "not to engage" with male screws of the bolt (non-constraint body) 9. This function provides easy attachment/detachment for the nut.