It is an object of this invention to provide a compact high-power alternator for a vehicle. An alternator for a vehicle includes a stator serving as an armature, and a rotor serving to generate a magnetic field. The rotor includes a pole core having a cylindrical portion, a yoke portion, and a claw-like magnetic pole portion. The stator includes a stator iron core, an armature coil, and an insulator. A ratio L1/L2 of an axial-direction length L1 of the stator iron core to an axial-direction length L2 of the pole core is in a range of 0.7 to 1.0.