To provide a bolt tightening tool and a bolt tightening method capable of preventing a tool body from being rotated together with an input shaft.

This bolt tightening tool 10 comprises a tool body 11, an input shaft 12 installed at one end part of the tool body 11 and rotated around the axis thereof, and an output shaft 13 installed at the other end part of the tool body 11 and rotated around the axis thereof by the rotation of the input shaft 12. The rotating torque of the output shaft 13 is increased more than a rotating torque inputted into the input shaft 2. An arm 15 projecting in the direction crossing the input shaft 12 and the output shaft 13 is installed on the outer peripheral part of the tool body 11.