To provide a fastening tool capable of surely rotating a lock nut and a coupler C to fasten them.

This fastening tool is composed of a forward and reverse driving motor 2, a rotating mechanism, a gear mechanism and a stopping mechanism. In the rotating mechanism, a rotary gear 5 is coaxially held between a pair of upper and lower slide metals 6 curved by cutting their front surfaces by approximately 70-80°, and rotated while sliding thereon, and a supporting member 7 is mounted on the rotary gear 5. A collet 8 having an internal size corresponding to the lock nut R and the coupler C of a joint fitting of various reinforcing bars F different size is fitted to the supporting member 7.