PURPOSE: To reduce the inertia of a rotor and to improve its responding proper ties by providing a thin superconducting material having the width of an electri cal angle of 90° for each electrical angle of 180° on a rotor yoke composed of magnetic material facing a stator winding.

CONSTITUTION: Stator windings 2A-2C are wound round slots of the inner periph eral face of a stator to connect between terminals A, A', B, B', and C, C', respectively. A thin superconducting material 9 having the width 90° of an electrical angle for each electrical angle of 180° is provided on a cup-shaped rotor yoke 8 composed of magnetic material facing said stator windings 2A-2C. A magnetic flux to said rotor yoke caused by a magnetic field generated by said stator windings due to the Meissner current of said superconducting mate rial does not pass through a part, where the superconducting material exists, but centers on a part, where there is no such material, so that the quantity of magnetic energy stored in an air gap differs according to the place. Thus, a rotor having a large inertia becomes unnecessary and responding properties of an apparatus are improved. Therefore, said apparatus is suitable for OA equipment and information equipment.