A fastening device includes a shaft having a longitudinal axis, a fixing member configured to fix the shaft to an endoscope by being attached to the endoscope, a staple unit attached to a distal end of the shaft and rotatable around the longitudinal axis, an operation unit provided at a proximal-end portion of the shaft, and a rotation restriction mechanism configured to restrict rotation of the staple unit around the longitudinal axis with respect to the endoscope due to an operation of the operation unit.