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 to be installed in a vehicle is structured such that the motor (133) is accommodated in a cylindrical space of a spool (132) and that the ratio of the outside diameter D1 of a motor housing (133a) of the motor (133) relative to the outside diameter D2 of a spool outer periphery (132a) of the spool (132), (D1/D2), is set to be 0.8 or less.