A rotation detecting device includes rotation detecting device includes a first supporting member and a second supporting member disposed around an axial line. The rotation detecting device further includes a pair of magnetic field generating portions for generating a magnetic field in a region between the first supporting member and the second supporting member. The rotation detecting device further includes at least one magnetic field detecting portion attached to the second supporting member for detecting the magnetic field. The rotation detecting device further includes a first magnetic member for covering one end portion of the magnetic field detecting portion and a second magnetic member for covering the other end portion of the magnetic field detecting portion.