A rotational angle detection device enables rotational angle detection for rotational angles ranging from 0° to 360° with an extremely simple configuration. The rotational angle detection device includes: a rotating magnet 12, which is mounted on one end of a rotating shaft 11; a four-pole auxiliary magnet 14, which is mounted on a substrate 13 opposed to the rotating magnet 12 without contact; and a magnetic sensor 15, which is mounted on the opposite side of the substrate 13. The four-pole auxiliary magnet 14 converts one rotating magnetic field of 0° to 360° provided by the rotating magnet 12 in a region where the magnetic sensor 15 is positioned into another rotating magnetic field of 0° to 180°.