A rotary element assembly structure in which a rotary element 53 is assembled to a tip side of a hollow shaft B1 provided upright on a pedestal 60. A cylindrical portion 53c for fitting with the hollow shaft is provided to a rotation center portion of the rotary element. In the cylindrical portion, a hook strut portion 53s that has a hook portion 53f overhanging in the radial direction at its tip side and that extends substantially in the fitting direction is arranged. A base end portion of the hook strut portion and a base end portion of the cylindrical portion are coupled with each other by a resin-made thin plate portion 53p. A small diameter portion B1s which is smaller in diameter than a hollow portion B1d of the hollow shaft is provided on the tip side of the hollow shaft. The radius of the overhang tip portion of the hook portion is set to be greater than the radius of the small diameter portion of the hollow shaft, and to be smaller than the radius of the hollow portion. By fitting the hollow shaft with the cylindrical portion of the rotary element, the hook portion on the tip side of the hook strut portion is pushed into the hollow portion via the small diameter portion on the tip side of the hollow shaft. Thereby, the hook portion engages with a stepped portion formed by the hollow portion and the small diameter portion.