A rotation angle detector has a rotation angle sensor and a microcomputer, where the sensor detects a change of a magnetic field of a magnet that rotates with a shaft. The microcomputer has a first corrector for correcting a pre-correction mechanical angle, based on a first correction value that corrects an error due to an assembly process for assembling the magnet and the sensor, and a second corrector for correcting the pre-correction mechanical angle based on a second correction value that corrects an error due to a spill magnetic flux that is generated by a supply of an electric current to a winding wire. In such configuration, the rotation angle detector may correct the error due to the spill magnetic flux, thereby enabling an accurate detection of a mechanical angle.