A rotation angle sensor includes a first and a second detection gears rotatable with a steering shaft, a first detector configured to detect a rotation angle of the first detection gear, a second detector configured to detect a rotation angle of the second detection gear, a processor configured to calculate a rotation angle of the steering shaft based on the rotation angles detected by the first and second detectors and a discriminator configured to discriminate whether the rotational angle of the steering shaft, calculated by the processor, falls in an abnormal value, wherein a least common multiple between a cycle of the first detector and a cycle of the second detector is greater than a steering angle measurement range.