PURPOSE: To limit a belt winding speed to a value within a given range by a method wherein, with the increase of the rotation speed of a belt winding shaft, resistance exerted on vanes by liquid in a closed chamber is increased and the further increased rotation speed of the winding shaft is blocked.

CONSTITUTION: The right end part 24 of a seat belt winding shaft 18 is inserted in the casing 28 of a winding shaft rotation decelerating device 26 and vanes 30 rotated within the casing 28 are fixed to the end part 24 of the winding shaft. The casing 28 forms a closed chamber and the interior thereof is filled with liquid 32. While the rotation speed of the winding shaft 18 is low, resistance exerted on the vanes 30 by the liquid 32 is low and the winding shaft 18 is lightly rotated. However, with the increase of the rotation speed of the winding shaft 18, resistance exerted by the liquid 32 is gradually increased. When the rotation speed of the winding shaft 18 exceeds a given speed, resistance exerted on the vanes 30 by the liquid 32 is eminently increased and the excessively high rotation speed of the winding shaft 18 is prevented.