A tightening system for tightening a fastening member includes a driving section producing torque for tightening the fastening member, an extension bar coupled to the driving section and rotating by the torque, a spindle to which the torque is transmitted via the extension bar and which is connected to the extension bar such that the spindle moves forward or backward in a rotational axis direction of the extension bar with respect to the extension bar, a socket provided at a tip of the spindle and engages in the fastening member, a spindle case housing the spindle and helically spline-engaging with the spindle along a rotational axis direction of the spindle, an outer case housing the spindle case, and a switch for switching between held and separated states between the spindle case and the outer case.