Disclosed is an angle sensor, which is capable of accurately measuring an angle of a steering shaft by selecting an optimum gear ratio between main and subsidiary gears constituting the angle sensor, measures a rotation amount of a vehicle steering shaft, and includes a ring-shaped main gear rotating along with the steering shaft, a first subsidiary gear rotated by rotation of the main gear and having a magnet coupled to a surface thereof, and a magnetic device detecting a change in magnetic field of the magnet and outputting data on a rotation amount, so that the first subsidiary gear rotates four times while the main gear rotates once, and a value of a scale factor is minimized during a process of acquiring valid data for measuring the angle of the main gear and thereby an error of a measured value is minimized.