A method of using a rotation angle detection device, the rotation angle detection device comprising: a correction-object driven gear that is a driven gear meshing with a main driving gear that is provided to a rotation shaft so as to be integrally rotatable with the rotation shaft; 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 is configured to compute a driven-side rotation angle, which is a rotation angle of the correction-object driven gear, based on the electrical signal generated by the first sensor, wherein the number of teeth of the correction-object driven gear is the number of teeth obtained by dividing the number of teeth of the main driving gear by an integer,, the method comprising: obtaining a correction angle which is a predetermined deviation in a predetermined angle domain obtained as an average value in which deviations of a number equal to the integer and corresponding to the same relative rotation angle is averaged, so as to be deviation in an angle domain of 0 to 360 degrees, the predetermined deviation is a deviation obtained in the predetermined angle domain and is deviation between the driven-side rotation angle and an actual rotation angle of the correction-object driven gear, and the driven-side rotation angle is obtained over a full angle domain when the correction-object driven gear is rotated for a number of times equal to the integer, wherein the correction angle is obtained taking into consideration not only deviation in an angle domain of 0 to 360 degrees but also deviation in an angle domain exceeding 360 degrees, storing, by the electronic control unit, the correction angle, rotating the correction-object driven gear so that the first sensor generates the electrical signal, computing, by the electronic control unit, the driven-side rotation angle, and correcting, by the electronic control unit, the driven-side rotation angle when computing the driven-side rotation angle, by using the correction angle. a correction-object driven gear that is a driven gear meshing with a main driving gear that is provided to a rotation shaft so as to be integrally rotatable with the rotation shaft; 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 is configured to compute a driven-side rotation angle, which is a rotation angle of the correction-object driven gear, based on the electrical signal generated by the first sensor, wherein the number of teeth of the correction-object driven gear is the number of teeth obtained by dividing the number of teeth of the main driving gear by an integer,, the method comprising: obtaining a correction angle which is a predetermined deviation in a predetermined angle domain obtained as an average value in which deviations of a number equal to the integer and corresponding to the same relative rotation angle is averaged, so as to be deviation in an angle domain of 0 to 360 degrees, the predetermined deviation is a deviation obtained in the predetermined angle domain and is deviation between the driven-side rotation angle and an actual rotation angle of the correction-object driven gear, and the driven-side rotation angle is obtained over a full angle domain when the correction-object driven gear is rotated for a number of times equal to the integer, wherein the correction angle is obtained taking into consideration not only deviation in an angle domain of 0 to 360 degrees but also deviation in an angle domain exceeding 360 degrees, storing, by the electronic control unit, the correction angle, rotating the correction-object driven gear so that the first sensor generates the electrical signal, computing, by the electronic control unit, the driven-side rotation angle, and correcting, by the electronic control unit, the driven-side rotation angle when computing the driven-side rotation angle, by using the correction angle. obtaining a correction angle which is a predetermined deviation in a predetermined angle domain obtained as an average value in which deviations of a number equal to the integer and corresponding to the same relative rotation angle is averaged, so as to be deviation in an angle domain of 0 to 360 degrees, the predetermined deviation is a deviation obtained in the predetermined angle domain and is deviation between the driven-side rotation angle and an actual rotation angle of the correction-object driven gear, and the driven-side rotation angle is obtained over a full angle domain when the correction-object driven gear is rotated for a number of times equal to the integer, wherein the correction angle is obtained taking into consideration not only deviation in an angle domain of 0 to 360 degrees but also deviation in an angle domain exceeding 360 degrees, storing, by the electronic control unit, the correction angle, rotating the correction-object driven gear so that the first sensor generates the electrical signal, computing, by the electronic control unit, the driven-side rotation angle, and correcting, by the electronic control unit, the driven-side rotation angle when computing the driven-side rotation angle, by using the correction angle.