A magnetism sensing element is provided to be rotatable relative to permanent magnets and outputs a magnetic force detection value, which corresponds to a perpendicular component of magnetic flux. A Hall IC calculates and outputs an output voltage, which corresponds to a relative rotational angle between the permanent magnets and the magnetism sensing element, based on the magnetic force detection value outputted by the magnetism sensing element. The Hall IC calculates the output voltage as V2=k×arcsin(V1/(VM+α))+Voffset, in which V1, V2, VM, k, α and Voffset indicate the magnetic force detection value, the output voltage, a maximum value of the magnetic force detection value, a gain, a predetermined value and a predetermined offset value.