Abstract: A generator has a rotor attached to one of a pair of spoke suspending hub flanges rotatably mounted on an axle of a bicycle and a stator fixedly mounted on the axle. The generator includes a hub drum having a disc portion and a rim portion, the disc portion being concentrically fixed on an outer surface of the one hub flange and formed at its central portion with a through hole through which the axle is rotatably inserted, the rim portion being integrally formed on an outer circumferential edge of the disc portion so as to project in the direction of an outer surface of the disc portion. An annular magnet assembly constituting the rotor is attached on an inner circumferential surface of the rim portion.