A sealing structure has a permanent magnet and a magnetic-flux guiding member for guiding magnetic flux of the permanent magnet to a brake shaft of a brake rotating member. The magnetic-flux guiding member surrounds an outer periphery of the brake shaft so as to form a sealing gap around the brake shaft. The sealing gap is communicated to a fluid chamber, in which magnetic viscous fluid is filled. A fluid sealing member is provided at the brake shaft at a housing outer side of the magnetic-flux guiding member to form an intermediate fluid chamber, in which an intermediate fluid made of non-magnetic liquid is filled.