There is provided a rudder angle sensor that is attached to a steering shaft for measuring an absolute angle of the steering shaft and includes: a coarse signal measuring mechanism and a fine signal measuring mechanism which measure a rotation angle of the steering shaft which rotates a predetermined number of times, the coarse signal measuring mechanism outputting a coarse signal and the fine signal measuring mechanism outputting a fine signal; and a calculating mechanism which calculates the absolute angle of the steering shaft based on coarse signal measurement data and fine signal measurement data converted from the coarse signal and the fine signal respectively, wherein the calculating mechanism is constituted by including an abnormal data detecting and eliminating unit which detects and eliminates abnormal data deviating from a predetermined range from the measurement data.