An AC alternator for a vehicle has a rotor having a pair of pole cores, and a stator. A plurality of claw magnetic pole parts are formed in the pole cores. A permanent magnet is placed between a pair of the claw magnetic pole parts so as to prevent leakage of magnetic flux through the area between the adjacent claw magnetic pole parts. The stator and the rotor are placed in the AC alternator so that the stator faces the rotor at a spacing of predetermined intervals. Flanges are formed at both sides of each of the claw magnetic pole parts. The presence of the flanges fixes the permanent magnets to the pole cores, and prevents them from moving toward the outer radius direction measured from the rotary shaft of the rotor. In particular, the thickness of each of the flange parts is increased from the front end part to the bottom part thereof.