To make a small-sized efficient magnetic circuit obtainable between an eddy-current reduction gear and a braking drum by a combined body of block-shaped (rectangular parallelepiped) magnets with a ferromagnetic member.

A guide pipe 18 consisting of a nonmagnetic material is coaxially arranged in a braking drum 13 coupled with a rotating shaft 4. A magnet supporting pipe 19 consisting of a nonmagnetic material is supported by the guide pipe 18, in such a state that the supporting pipe 19 is movable in the axial direction by slipping the supporting pipe 19 on the guide pipe 18. Many magnets 20 are supported on the outer peripheral wall of the supporting pipe 19 at regular intervals in the peripheral direction. The base-side end face 21a of a ferromagnetic member 21 is coupled with the magnetic pole face of each magnet 20 at both end in the peripheral direction. An actuator 35, which drives the magnet supporting pipe 19 in both the forward and backward directions, is provided to a braking position where the front end face 21b of the ferromagnetic member 21 faces opposite to the inner peripheral surface of the braking drum 13 and to a non-braking position where the ferromagnetic member 21 retreats outward from the drum 13.