Seat belt retractor

In order to provide a technology effective for a 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 size (L1) of a motor housing (133a) of the motor (133) in the axial direction and the size of a spool outer periphery (132a) of the spool (132) in the width direction are broadly equal to each other.