Seat belt retractor

In order to provide a motorized seat belt retractor (1) in which a power transmission mechanism (8) can be prevented from being subjected to a load torque exceeding a preset value during the driving of a motor (6), when the load torque in the belt winding direction to be transmitted to an upper-side connecting gear (29) is smaller than the preset value, ratchet teeth (28b, 29b) engage each other so that the driving torque of the motor (6) is transmitted from a lower-side connecting gear (28) to the upper-side connecting gear (29). When the load torque exceeds the preset value, ratchet teeth (28b) slip over the slopes of ratchet teeth (29b) so that the lower-side connecting gear (28) rotate relative to the upper-side connecting gear (29). The relative rotation of the ratchet teeth (28b) relative to the ratchet teeth (29b) cancels the engagement between the ratchet teeth (28b, 29b) so that the upper connecting gear (29) is never subjected to a load torque exceeding the preset value.