A torsion bar capable of torsional deformation, a ring arranged at a shaft end portion of a spool and connected rotatably integrally with an outer peripheral portion on the other side in an axial direction of the torsion bar, a plastic-deformable plate member having one end side fixed to an outer peripheral portion of the ring and the other end side being a free end in the vicinity of an inner peripheral portion of the shaft end portion of the spool, and a projecting portion which projects from an inner peripheral surface of the spool, which is brought into contact with the other end of the plate member after the torsion bar starts rotating integrally with the spool and an impact absorbing load of the torsion bar becomes constant due to torsional deformation, and which imparts a load for plastically deforming the plate member in further rotation of the spool.