PURPOSE: To detect the wound volume of webbing easily and accurately by having epicyclic gears installed on a retractor shaft engaged with fixed and rotary internal gears each having different number of teeth, and installing a switch operating part in the rotary internal gear.

CONSTITUTION: When a retractor shaft 10 rotates, a rotating plate 14 affixed thereto rotates. A plural number of epicyclic gear 40 rotatingly fixed on both ends of the rotating plate 14 rotates, engaged with fixed and rotary internal gears, 20 and 30, which are arranged adjacent to the axial direction of the retractor shaft 10. The rotary internal gear 30 then rotates relatively against the fixed internal gear 20, in proportion to the difference in the number of teeth. As the result, the rotation of the retractor shaft 10 is reduced at a specified reduction ratio and transmitted to the rotary internal gear 30. The rotational speed of the retractor shaft 10 is detected when a collar part 34 formed on the outer periphery of the rotary internal gear 30 abuts on the operating piece 82 of a limit switch 80.