To improve sensitivity to easily detect the motion of a to-be-detected object and assure low cost and also improved workability.

A first bias magnet 25a which generates a bias magnetic field toward a gear 11 having a first magnetic material and a magnetic material 45 which is arranged in the moving direction of the gear 11 opposed to the bias magnet 25a are provided and the magneto-resistive elements 16a1, 16a2 which show change of resistance depending on the condition change of the bias magnetic field corresponding to the movement of gear 11 are arranged at the plane almost perpendicular to the moving direction of the gear 11 in the bias field between the first bias magnet 25a and magnetic material 45.