A manufacturing method of a rotating element including a cylindrical fitting portion having a pair of circumferential end portions forming an axial slit between the circumferential end portions, the cylindrical fitting portion being serration-fitted to a rotating shaft, includes: clamping the fitting portion by a clamp jig so that the fitting portion is elastically deformed in an elliptical shape; forming a female serration arranged in an elliptical shape by use of a tool having an elliptical section, the female serration being formed on an inner peripheral surface of the fitting portion elastically deformed in the elliptical shape; and releasing the clamping by the clamp jig so that the arrangement of the female serration is changed to a perfect-circle shape from the elliptical shape.