A magneto-generator capable of suppressing temperature rise of an armature winding and ensuring an improved electricity generation efficiency includes a flywheel having a cylindrical peripheral wall formed with a flat bottom wall at one end thereof, a plurality of magnets disposed on and along an inner peripheral surface of the cylindrical peripheral wall, a generator coil disposed internally of the flywheel in opposition to the magnets for generating electricity under electromagnetic induction taking place between the magnets and the generator coil, ventilating through-holes formed in a joined portion of the cylindrical peripheral wall and the flat bottom wall so as to extend partially through both of them, and upstanding fins disposed on the bottom wall for generating forced air flows from interior to exterior of the flywheel through the ventilating through-holes upon rotation of the flywheel.