PURPOSE: To provide a fastening tool by which looseness of fastening can be reliably prevented and which can be inexpensively manufactured without requiring highly accurate work in the fastening tool using a bolt and a nut.

CONSTITUTION: A slit 1a is cut along the central axis from the tip of a bolt 1, and this slit 1a is arranged so as to divide a cross section of the bolt 1 in a radial shape. A central hole is bored along the central axis from a head part of the bolt 1, and the depth reaches a part having the slit 1a in a tip part, and is arranged so as to continue with the slit 1a. An inserting pin 4 is provided to be inserted in the central hole from the head part of the bolt 1, and since this tip is thrust in to the more tip side than a bottom part of the central hole, a tip part of the bolt 1 can be expansively opened in a radial shape. It is desirable that a nut whose inside diameter is expanded by the tip side when screw-fitted to the bolt is used as a nut 2.