A rotation angle detection device for detecting a rotation angle of a rotating body includes a magnet having poles arranged along a radial direction perpendicular to a rotation axis of the rotating body, the magnet being disposed to rotate together with the rotating body, a magnetic portion provided in a ring shape radially outward of the magnet, a plurality of gaps being formed in the magnetic portion at a plurality of locations along a circumferential direction, and a magnetic detection unit arranged in a particular gap of the plurality of gaps, the magnetic detection unit being configured to detect a tangential magnetic flux component in a tangential direction and a radial magnetic flux component in the radial direction of a magnetic field.