In this azimuth angle sensor, original magnetic detection data is sequentially acquired as data points in a triaxial coordinate system by detecting magnetism on three axes. An offset derivation device provided to the azimuth angle sensor derives offset values for correcting the original magnetic detection data and generating corrected magnetic detection data. The offset derivation device uses a plurality of data points in the original magnetic detection data or in the corrected magnetic detection data to derive provisional offset values, then derives the magnitude of magnetism on such an occasion on the basis of the original magnetic detection data and the provisional offset values. When the magnitude of magnetism thus derived is within a predetermined correction success range, the offset values are updated using the provisional offset values. When the magnitude of magnetism thus derived is not within the predetermined correction success range, the offset values are not updated.