With the light-emitting side focal point F1 of a three-dimensional elliptical reflecting body A being on the side of a rotating body, the light-receiving side focal point F2 being on the side of a fixed body, the condensing action from the F1 to F2 is used to configure an optical path from a rotating-side light-emitting element A set at F1, via a fixed-side elliptical partial mirror in which a portion of the three-dimensional elliptical body is used as a mirror face, to a fixed-side light-receiving element A set at F2, and to configure a simultaneous bidirectional single-channel optical path from a fixed-side light-emitting element A set at F1 of a three-dimensional elliptical reflecting body B, via a rotating-side elliptical partial mirror, to a rotating-side light-receiving element A set at F2.