A rotor resin assembly of a rotor of an electric motor includes a rotor magnet including a substantially cylindrical yoke molded from thermoplastic resin containing a soft magnetic material or ferrite and a resin magnet integrally formed in the outer circumference of the yoke, a position detection magnet arranged at one end in the axial direction of the rotor magnet, and a substantially doughnut-like rotor core arranged in the inner circumference of the rotor magnet and formed by laminating a predetermined number of electromagnetic steel plates. The rotor magnet, the position detection magnet, and the rotor core are integrally formed by resin. A shaft is inserted into an inner diameter section of the rotor core, and the rotor resin assembly is caulked by a jig to form a recess in the vicinity of the inner diameter sections of both end faces of the rotor core and assembled to the shaft.