Tightening system

A tightening system for tightening a fastening member, the system comprising:

• a driving section which produces torque for tightening the fastening member;
• an extension bar which is coupled to the driving section and rotated 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 which is provided at a tip of the spindle and engages in the fastening member;
• a spindle case which houses the spindle and makes helical spline engagement with the spindle along a rotational axis direction of the spindle;
• an outer case which houses the spindle case;
• and
• a switch for switching between a state where the spindle case is held to the outer case and a state where the spindle case is separated from the outer case.