Rotation angle detection apparatus includes a magnet provided in a main gear and a magnetism detector positioned to face to the magnet. Magnet rotates integrally with main gear and magnetism detector detects changes in magnetic field of magnet so a rotation angle of main gear is detected. Main gear includes bottomed recessed part and plurality of mounting pins arranged in recessed part. Magnet is inserted in recessed part wherein a part of magnet protrudes from an upper end of the recessed part. Magnet inserted in recessed part is pressed against main gear by an elastically deformable retaining plate made of non-magnetic material. An outer portion of retaining plate includes plurality of mounting holes. Magnet inserted in recessed part is pressed against main gear by elastically deformed retaining plate and mounting pins inserted in mounting holes are thermally caulked wherein magnet is secured to the main gear.