PURPOSE: To obtain a bolt fastening tool which can prevent a mis-fastening or an accompanying rotation of a bolt or the like, by providing a bit for fixing to engage to the groove of the bolt or the like, in a socket rotated by a gear.

CONSTITUTION: When a nut 5 is fastened by a socket 6, a bit 7 for bolt fixing provided in the socket 6 is engaged to the groove of a bolt or the like 1. This bit 7 for fixing is fixed to a bit slide tool 18 through a bit return spring 15, only in the rotating direction by a rectangular shaft or the like. It is made movable in the lateral direction through the bit return spring 15. And even though the nut 5 is fastened and rotated by the socket 6, and advances in the lateral direction, the bit 7 for fixing follows by the bit return spring 15, and the bolt or the like 1 is fixed constantly. Consequently the nut 5 is not rotated together with the bolt or the like 1, and can be fastened securely.