A rotation angle detecting device includes a housing, a rotating shaft rotatably held by the housing, a driving gear fixed to the rotating shaft, a driven gear engaged with the driving gear, a magnetic field generating part fixed in the driven gear, having a permanent magnet, and configured to generate a magnetic field, a magnetic sensor element configured to detect a change in the magnetic field due to a rotational movement of the magnetic field generating part, and a signal processor configured to process a signal from the magnetic sensor element for detecting a rotation angle of the rotating shaft. Each of teeth of the driving gear has a first circumferential width, each of teeth of the driven gear has a second circumferential width, and the second circumferential width is greater than the first circumferential width.