A rotation angle detection device includes a correction-object driven gear that is a driven gear meshing with a main driving gear, a first sensor that is configured to generate an electrical signal based on rotation of the correction-object driven gear, and an electronic control unit that computes a driven-side rotation angle based on the electrical signal. The electronic control unit is configured to store a correction angle used to correct the driven-side rotation angle when computing the driven-side rotation angle. The correction angle is a predetermined deviation in a predetermined angle domain obtained as an average value in which deviations of the number equal to the integer and corresponding to a same relative rotation angle is averaged, so as to be deviation in an angle domain of 0 to 360 degrees.