External magnetic members are provided outside the outer periphery of a rotor. When the rotor is positioned around its turning angle of 70 degrees, part of the magnetic fluxes generated by magnets pass through the external magnetic members. This makes it possible to reduce the magnetic fluxes passing through Hall ICs around the turning angle of 70 degrees, thereby linearizing the change characteristic of the magnetic flux density. Thus, it is possible to generally linearize the change characteristic of the density of the magnetic fluxes through the Hall ICs 7, with the detectable angle range kept wide.