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.