To improve stabilization of a torque in a low speed region and radiating effect of a frictional heat so as to suppress torque ripple at the time of low speed rotation by changing electromagnetic powder into a magnetic fluid.

When a magnetic fluid 12 formed by mixing magnetic powder with a fluid having viscosity is pushed from a magnetic fluid filling port 11a2, the magnetic fluid 12 flows down to a space 16b through a narrow space 16B2 from right and left spaces. The magnetic fluid 12 is held in the narrow space 16B2 of an annular protruding part and in a narrow clearance 16B1 between 11c1 and 15b1 by action of a magnetic flux 2 of permanent magnets 20a and 20b, and the role for sealing is executed. Magnetic powder in the magnetic fluid 12 of a part where the flux 2 of the permanent magnets 20a and 20b is not acted is collected in spaces 16c1 and 16c2 around a cylinder 15a when current-carrying is carried out on a coil, deviation of magnetic powder components is suppressed in a low level, and fluctuation of a torque is suppressed in a low level. It is thus possible to prevent deterioration caused by abrasion of sealing, and it is possible to use a device semi-permanently.