Abstract: An absolute steering angle of a steering shaft for a vehicle is measured using rotatable bodies that rotate together with the steering shaft at respective predetermined rotation ratios. A ?M? value is obtained by measuring a relative rotational angle ?? of a first rotatable body and a ?M? value is obtained by measuring the relative rotational angle ?? of a second rotatable body. ?C's are obtained by calculating relative rotational angles ?'s of the second rotatable body corresponding to the ?M? value, using the relation between ?? and ??. A frequency i-value of the first rotatable body is obtained by comparing the ?Cs to the ?M? value. An absolute steering angle ?1 of the steering shaft is obtained based on the relation between absolute rotational angles ? and ?, after ? is obtained using the i-value.