The winder comprises a mandrel 4 rotatively mounted in a support 1 and around which the strap of the safety belt is wound. The mandrel is biased by a torsion spring in the winding direction and a locking device, adapted to lock the mandrel 4 against rotation and prevent the unwinding of the strap, is provided. The locking device comprises a first element 9 rigid with the support 1 and adapted to cooperate, under the control of an acceleration detector, with a second element 7 which is driven by the mandrel 4 for locking the mandrel against rotation. One of the first and second elements comprises projections for engagement in recesses provided in the other element 9. Abutment devices 10 are provided on one of the first and second elements for preventing, in normal use of the winder, the full engagement of the projections in the recesses. The abutment devices are destructible in the event of impact resulting in a sudden pull on the strap.