A self-locking seat belt retractor includes a blocking system for a seat belt winding shaft that is actuated in an at least vehicle-sensitive manner, in which, in the event of triggering, a sensor mass mounted in a vehicle-sensitive manner moves a locking lever with its engagement tip into a trigger position in engagement with the toothing of a control disk, as the result of which the control disk in its joint rotational motion with the belt winding shaft is stopped and the blocking system is activated. The sensor mass is situated in a support part on a contact surface, and when vehicle accelerations occur the sensor mass in the support part is movable on the contact surface by inertial control from a neutral position into the trigger position. The support part together with the sensor mass situated therein is situated in a housing that includes the control disk. The housing with the support part and the sensor mass situated therein is fixable on the seat belt retractor in various rotational angle positions with respect to the control disk.