A rotor structure comprising magnets for suppressing the leakage of a magnetic flux between the side faces of the adjacent claw-like magnetic poles and metal holding members each of which covers the magnets together with the claw-like magnetic pole and radiates heat transmitted to the claw-like magnetic pole at the time of power generation. Therefore, the quantity of heat radiated to the peripheral portions of the magnets is increased and the coolability of the magnets can be greatly improved.