To provide a detector which can accurately generate signals corresponding to the prescribed position (angle) such as surface unevenness of a moving body made of a magnetic substance.

This detector is equipped with a magnet 4 generating a magnetic field, an rotor of magnetic substance installed with a prescribed interval to the magnet 4 to vary the generated magnetic field, and a gigantic magnetic resistance element 10 to detect the varied magnetic field where a uniform resistance varying range is set as the operating range for any of the magnetic field varying directions. From the center of the magnet 4, the center of the magnetic sensitive surface of the gigantic element 10 is dislocated parallel with the plane in which the moving direction of the rotor 2 is included.