Seat belt retractor

This seat belt retractor comprises: a winding drum around which a webbing is wound; a transmission member for transmitting rotational drive force, the transmission member being provided on the same axis as the rotational axis of the winding drum and having a plurality of convexities protruding radially outward formed at a predetermined circumferential pitch in the outer periphery of at least one end; and a fitting member into which the end of the transmission member having the convexities is fitted, the fitting member having fitting parts in which the convexities are fitted; the convexities being formed in a cross-sectional trapezoid shape so that the angle of incline relative to the radial direction of one of the two side surfaces in the circumferential direction is smaller than the angle of incline relative to the radial direction of the other side surface, and the one side surface being configured so as to bear a greater load by the rotational drive force transmitted during an emergency than the load borne by the other side surface via the fitting member.