A rotary angle detecting device comprising: a main driving gear configured to rotate while being directly or indirectly connected to a detection target rotating about a predetermined axis; a first driven gear and a second driven gear, which are configured to rotate while meshing with the main driving gear; a magnet provided at the center of each of the driven gears (13, 14); a holder member including a first insertion hole which allows the detection target to be inserted therethrough and rotatably supports the main driving gear by means of the first insertion hole and a support portion (30e, 30f) which rotatably supports the driven gears (13, 14), the support portion (30e, 30f) being provided with an opening which faces the magnet; a first cover member including a second insertion hole which allows the detection target to be inserted therethrough and a first space which is formed between the first cover member and the holder member when the first cover member is assembled to one side of the holder member so as to rotatably sandwich the main driving gear and the driven gears (13, 14) and to accommodate the driven gears (13, 14), a part of the main driving gear being disposed in the first space; a magnetic detection element configured to face the magnet with the opening interposed therebetween so as to detect a variation in magnetic field in accordance with a rotation of the magnet; a circuit board having the magnetic detection element mounted thereon; and a second cover member including a third insertion hole which allows the detection target to be inserted therethrough and a second space which is formed between the second cover member and the holder member when the second cover member is attached to the other side of the holder member so as to accommodate the circuit board,wherein the opening is closed by the circuit board, characterized in that the main driving gear is provided with a first stage gearing and a second stage gearing, the two stage gearings (12b, 12c) being arranged concentrically to each other and having different diameters and a different number of teeth; the first driven gear and the second driven gear have the same number of teeth; the first driven gear is in engagement with the first stage gearing; and the second driven gear is in engagement with the second stage gearing. a main driving gear configured to rotate while being directly or indirectly connected to a detection target rotating about a predetermined axis; a first driven gear and a second driven gear, which are configured to rotate while meshing with the main driving gear; a magnet provided at the center of each of the driven gears (13, 14); a holder member including a first insertion hole which allows the detection target to be inserted therethrough and rotatably supports the main driving gear by means of the first insertion hole and a support portion (30e, 30f) which rotatably supports the driven gears (13, 14), the support portion (30e, 30f) being provided with an opening which faces the magnet; a first cover member including a second insertion hole which allows the detection target to be inserted therethrough and a first space which is formed between the first cover member and the holder member when the first cover member is assembled to one side of the holder member so as to rotatably sandwich the main driving gear and the driven gears (13, 14) and to accommodate the driven gears (13, 14), a part of the main driving gear being disposed in the first space; a magnetic detection element configured to face the magnet with the opening interposed therebetween so as to detect a variation in magnetic field in accordance with a rotation of the magnet; a circuit board having the magnetic detection element mounted thereon; and a second cover member including a third insertion hole which allows the detection target to be inserted therethrough and a second space which is formed between the second cover member and the holder member when the second cover member is attached to the other side of the holder member so as to accommodate the circuit board,wherein the opening is closed by the circuit board, characterized in that the main driving gear is provided with a first stage gearing and a second stage gearing, the two stage gearings (12b, 12c) being arranged concentrically to each other and having different diameters and a different number of teeth; the first driven gear and the second driven gear have the same number of teeth; the first driven gear is in engagement with the first stage gearing; and the second driven gear is in engagement with the second stage gearing.