A seatbelt retractor includes a rotation transfer mechanism configured to transfer rotation of an adjustment gear to a sensor housing to swing the sensor housing in the front-back direction of a vehicle. The turning axis of the adjustment gear and the swing axis of the sensor housing intersect each other at a predetermined angle with the swing axis of the sensor housing arranged in a horizontal direction with respect to the left-right direction of the vehicle. An inertial element support surface of the sensor housing is kept horizontal by swing motion of the sensor housing in the front-back direction of the vehicle, even if a seat back is tilted in the front-back direction of the vehicle.