Shear ribs are formed at a bottom portion of a fitting hole of a spool for positioning a lock base, and respective end portions of the positioning portion abuts on a shear rib. Further, ribs are formed between the shear ribs. When the spool rotates in an unwinding direction with respect to the lock base, the end portion of the positioning portion deforms and breaks a shear rib, a rib and then another shear rib in that order. By being able to combine the effects of energy absorption due to deformation of the ribs with that of energy absorption due to twisting deformation of the torsion bar, a force limiter load immediately increases as soon as the spool starts to rotate.