Abstract: An eddy current retarder includes a rotor (3) fitted over a rotating shaft (1) at one end thereof and a support ring (7) around which a plurality of permanent magnets (10) are circumferentially provided in such a manner that the polarities of any adjacent magnets become opposite to each other. The support ring (7) is provided such that it is allowed to move back and forth in the axial direction of the rotating shaft (1) from a first position where the pole surfaces of the permanent magnets (10) fully confront the rotor (3) with a predetermined gap being secured therebetween to a second position where the former fully deviate from the latter.