To provide a bolt and nut tightening device which tightens or loosens the bolt and nut even when a space above a bolt fastening part is blocked.

The bolt and nut tightening device 1 has a base part mounted to a robot, a spindle part 3 provided so as to rotate by a tightening motor fixed on the base part, and an offset arm 4 provided so as to turn around a periphery 302 of the spindle part 3 by turning motor 22 fixed on the base part. A spindle socket 31 which is engaged with a bolt 50 is provided at a top part 301 of the spindle part 3, and rotates with the rotation of the spindle part 3. An offset socket 41 which is engaged with the bolt 50 is provided at the top part 401 of the offset arm 4. The offset socket 41 rotates by receiving the rotation of the spindle part 3 through a power transmission mechanism 42.