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 (100) to be installed in a vehicle is structured such that the ratio (V1/V2) of a volume V1 of a motor housing (133a) of a motor (133) to a volume V2 of a cylinder portion of a spool (132) is set to be within a retractor miniaturization adjustment range of the volume ratio in which the ratio (V1/V2) is about 40 percent or more and less than about 100 percent.