To reduce the drawing manhour, to reduce the peripheral space to be required, and to eliminate the interference with other stud bolts.

A polygonal blind hole 1b to be engaged with a nut is provided in a lower end of a socket 1 provided with a hole 1a to insert a drive bit on an upper end, two semi-cylindrical cover 4 are fitted to the periphery of the socket 1 in a non-turnable manner to each other, and two covers 4 can be attached/detached to/from each other with the prescribed distance therebetween. A half-split female screw 4b is provided in each lower end of two covers 4, a nut 6 in which a ring disk 7 having an inverse male screw 7a on the outer circumference is fixed to its lower end is installed in an attaching/detaching manner to/from the polygonal blind hole 1b in the inner space to be constituted by the lower end of the socket 1 and the covers 4, and an inverse female screw 4c to be constituted in bringing the covers 4 into close contact with the corresponding parts of the inverse male screw 7a on the inner circumference of the covers 4 is provided.