PROBLEM TO BE SOLVED: To provide a seat belt retractor driven by a motor capable of hindering load torque equal to or over the set value from acting on a power transmitting mechanism when the motor is put in operation. SOLUTION: The seat belt retractor is structured so that two sets of ratchet teeth 28b and 29b are engaged when the load torque in the belt takeup direction to be transmitted to an upper connection gear 29 is smaller than the set value and the drive torque of the motor 6 is transmitted from a lower connection gear 28 to the upper connection gear 29. When the load torque is equal to or above the set value, the ratchet teeth 28b slide on the slopes on the ratchet teeth 29b to generate relative rotation. By this relative rotation, the two sets of ratchet teeth 28b and 29b are disengaged, and the upper connection gear 29 is free of application of a load torque equal to or above the set value. COPYRIGHT: (C)2005,JPO&NCIPI