Fastening device and method of controlling fastening device

A method of controlling a fastening device that inserts a washer into a bolt protruding from a member to be fastened and fastens a nut to the bolt, the fastening device including

• a nut holding mechanism that has a holding portion formed in a cylindrical shape extending along an axis and holding the nut at a tip portion, and a rotating portion that rotates the holding portion around the axis,
• a movement mechanism that is attached to the nut holding mechanism and is movable with respect to the nut holding mechanism along the axis,
• a gripping mechanism that is attached to the movement mechanism and brings a pair of gripping portions close to or away from each other along a direction orthogonal to the axis, and
• a positioning mechanism that positions the nut holding mechanism at a predetermined position in a three-dimensional space to have a predetermined posture, the method comprising:
• a gripping step of causing the gripping mechanism to bring the pair of gripping portions close to each other in front of the tip portion along the axis to grip the washer in a state where the nut holding mechanism holds the nut;
• an insertion step of causing the positioning mechanism to insert the bolt into the washer;
• and
• a fastening step of causing the movement mechanism to bring the nut in contact with the bolt in a state where the gripping mechanism grips the washer, and causing the rotating portion to rotate the holding portion around the axis to fasten the nut to the bolt in a state where the nut is in contact with the bolt.