An increase of the magnetization current can be prevented during demagnetization and magnetization, and a variable speed operation can be achieved at a high power output over a wide range of from a low speed to a high speed. A rotor (1) is configured by a rotor core (2), a variable magnetic force magnet (3) and a fixed magnetic force magnet (4). A variable magnetic force magnet (3) and a fixed magnetic force magnet (4a) are overlapped in the magnetization direction thereof to form a series of magnets. The series of magnets is located within the rotor core at a position where the magnetization direction is in the direction of a d-axis. On either side of the series of magnets of the variable magnetic force magnet (3) and the fixed magnetic force magnet (4a), fixed magnetic force magnets (4b, 4b) are located at a position where the magnetization direction is in the direction of the d-axis. When the flux linkage of the variable magnetic force magnet is reduced, a current of an armature coil allows a magnetic field to act in the reverse direction to the magnetization direction of the variable magnetic force magnet. When the flux linkage of the variable magnetic force magnet is increased, a current of the armature coil allows a magnetic field to act in the same direction as the magnetization direction of the variable magnetic force magnet.