A rotation angle detecting device is rotated by a rotating object via a gear mechanism to detect a rotation angle of the rotating object. The rotation angle detecting device includes a housing, a magnet rotor unit having a permanent magnet and a central hole, a magnetic sensor unit including a pair of magnetic sensor elements each of which detects magnetic flux density of a magnetic field generated by the permanent magnet in a direction different from the other, and a signal processor that calculates a rotation angle of the rotating object from the magnetic flux density. The magnetic rotor unit includes a mechanism for changing the magnetic flux density as the number of turns of the magnet rotor unit changes.