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

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 (100) is structured such that a motor (133) is accommodated in a hollow cylindrical space of a spool (132) and such that the correlation among the volume V1 of the motor housing (133a) of the motor (133), the volume V2 of the cylinder portion of the spool (132), and the storage volume V3 within the seat belt retractor housing is set to achieve the equation V3 < V1 + V2.