A housing forms a fluid chamber in an inside of the housing. A magneto-rheological fluid includes a plurality of magneto-rheological particles dispersed in the magneto-rheological fluid and is filled in the fluid chamber. A partition structure partitions the fluid chamber between a radially outer region, in which the magnetic portion is exposed, and a radially inner region, which is located on a radially inner side of the radially outer region in a radial direction of a rotatable brake body, during rotation of the rotatable brake body.