Seat belt retractor, seat belt apparatus, and vehicle with a seat belt apparatus

In order to provide a technology effective for miniaturization of a seat belt retractor to be installed in a vehicle, a retractor (130) of a seat belt apparatus is structured such that a Hall sensor (134) and a magnetic disc (135) as a means for detecting the position of a rotor in a motor (133) are also used as a means for detecting the amount of winding or unwinding of a seat belt (110).