An electric rotary machine is provided with a stator having a stator core of laminated iron plates with a given lamination thickness in an axial direction of the machine and a stator winding wound around the core and a rotor rotatably disposed inside the stator, having a rotor core and a rotor winding. The axial length of the stator core is selected with respect to that of the rotor core in such a way that, when the rotor is thermally extended, the end of the rotor core never extends beyond the end of the stator core. With such a constructional arrangement, there is eliminated an excessive temperature rise at the ends of the stator core, and prevented an insulation deterioration of the stator winding and the burning of the stator core.