To provide a seatbelt retractor in which a reel is rotated automatically in a belt extraction direction by an ultrasonic motor following the extraction of a seatbelt.

A shaft 30 is inserted in a reel 20 and both are connected by a tension spring 37. The projection part 104 of the shaft 30 is connected to an ultrasonic motor 24. A lock mechanism 22 consisting of a ratchet wheel 38, a pawl 58, a lock ring 44, a hook retainer 72, a hook 74 and a flywheel 90 are installed on the reel 20. A variable resistor 36 is installed between the flange 32 of the reel 20 and the flange 34 of the shaft 30. Its resistant value is changed by the phase difference of both. The ultrasonic motor 24 is operated so that this phase difference is within a prescribed value.