A rotary angle detecting device is provided which includes: a main driving gear configured to rotate while being directly or indirectly connected to a detection target rotating about a predetermined axis; a driven gear configured to rotate while meshing with the main driving gear; a magnet provided at the center of the driven gear; a magnetic detection element configured to detect a variation in magnetic field in accordance with a rotation of the magnet; a circuit board having the magnetic detection element; and the circuit board is separated from the gears by a holder member.