A method for initializing a rotational angle detector detecting rotational angles of two driven gears mated with a drive gear, rotated integrally with a detection subject, and obtaining rotational angle of the detection subject from the detected rotational angles. The method includes obtaining a first error and second error in each rotational angle of the driven gears when the drive gear is rotated in forward and rearward directions, obtaining an average value of the first and second errors, obtaining a difference between the average value and a theoretical rotational angle for the two driven gears when the rotational angle of the drive gear is 0°, and storing the difference as correction data added to each actual rotational angle of the two driven gears when obtaining the rotational angle of the detection subject.