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 motor (133) is accommodated in a cylindrical space of a spool (132) and is provided with a motor speed-reduction mechanism for reducing the rotational speed of the motor (133) so that the spool (132) is rotated in a state wherein the rotational speed of the motor (133) is reduced by the motor speed-reduction mechanism.