A motor-generator is disclosed, in which electromagnet coils are arranged at axially opposite ends of the stator, each to each end, while connections among winding sets are changed in response to the rpm of the rotor. This makes it possible to control the voltage, either at high speed or at low speed, thereby increasing the torque at low speed, or the torque on the rotor shaft at low speed and rendering the voltage stable. A rotor is comprised of a cylindrical magnetic path axially elongating to extent confronting the electromagnet coils, and a permanent-magnet member of more than one permanent-magnet piece extending axially over the magnetic path. The cylindrical magnetic path includes an N-pole annular portion confronting any one of the electromagnet coils, N-pole electromagnet magnetic paths stemming from the N-pole annular portion in opposition to N-poles of the permanent-magnet pieces, an S-pole annular portion arranged confronting another of the electromagnet coils, and S-pole electromagnet magnetic paths stemming from the S-pole annular portion in opposition to S-poles of the permanent-magnet pieces.