PURPOSE: To obtain an output torque by passing a magnetic flux generated by an armature coil only into a single loop by using the magnetic path of a closed circuit, and introducing a field magnetic flux also into the same loop.

CONSTITUTION: The outer peripheral rotary surfaces of rotors 4a, 4b are uniformly magnetized to N-, S-poles, and a closed magnetic path shown by arrows 24a, 24b are formed by armature cores 13a, 14a opposed to the outer periphery through an air gap of a predetermined distance. Here, when armature coils 5a, 6a wound on the cores 13a, 14a are energized, a magnetic path as shown by an arrow 23 is formed. Accordingly, the magnetic fluxes by the coils 5a, 6a and the magnetic fluxes by rotors 4a, 4b become the same direction at the upper sides of the cores 13a, 14a to increase magnetic flux amounts, and both the magnetic fluxes at the lower side are opposed to reduce the amounts. Accordingly, the rotors 4a, 4b are rotated by driving torque in the direction of an arrow L. The other armature is similarly operated.