To provide a restraint device with excellent comfortableness in wearing a belt and safety.

A seat belt retractor has a retractor locking part which blocks the seat belt from being pulled out of the belt retractor in the event of an accident, and a tightening device which tightens the seat belt in a hazardous situation. The tightening device includes a tightening pawl which, in a blocking position, blocks the belt retractor in the belt uncoiling direction and, in a release position, leaves the belt retractor unaffected. The tightening pawl is provided with a sensor which monitors the position of the tightening pawl and transmits a signal indicating the position of the pawl to a control device of the tightening device.