An alternator has a rotor rotated on its own axis to generate electric power in a stator. The rotor has pole cores with claw portions arranged along the circumferential direction, and a holder unit is disposed between two adjacent claw portions in each pair. Each holder unit has a magnet accommodating holder and a magnet covering holder. Each holder has one bottom wall, four side walls and one opening. The magnet accommodating holder accommodates a magnet. The magnet covering holder is placed between the claw portions and accommodates the magnet accommodating holder while covering the magnet exposed to the opening of the magnet accommodating holder. The magnet accommodating holder has convex portions existing in the respective side and bottom walls and being in elastic contact with the magnet.