A resin molded rotor includes a rotor, a main shaft, and a resin mold. The rotor is configured to hold a magnet. The main shaft is provided with the rotor mounted thereto and is configured to transmit power to the outside. The resin mold is configured to integrally cover the rotor and part of the main shaft on respective sides in an axial direction of the rotor. An O-ring is placed between the resin mold and the main shaft to seal between the resin mold and the main shaft.