A rotation detecting device includes first to fourth magneto-resistive elements that are provided on a first substrate to constitute a bridge circuit, a detection circuit provided on a second substrate, first to fourth wirings each connecting between the detection circuit and respective one of ends of the first to fourth magneto-resistive elements, first to fourth nodes provided on the second substrate, and first and second amplifiers provided on the second substrate. The first node combines signals on the first and second wirings. The second node combines signals on the third and fourth wirings. The first and second amplifiers amplify signals at the first and second nodes, respectively.