Abstract: A method of separating an oscillatory potential wave from an electroretinogram of a subject which contains an "a" wave, a "b" wave and said oscillatory potential wave that overlap each other. The electroretinogram ([1]) is produced by light stimulation to the retina of an eye of the subject, and is detected by a suitable device. The peak-to-peak distances of the detected electroretinogram are determined, and a mean period (T) of the oscillatory potential wave ([10]) to be separated from said electroretinogram is obtained. Then, a tentative start point (A) of the oscillatory potential wave is determined, and an OP-free waveform ([7], [9]) free of said oscillatory potential wave is obtained by obtaining moving averages of a first intermediate waveform ([6], [8]) which is determined based on the electroretinogram and the determined tentative start point (A). The oscillatory potential wave ([10]) is extracted by subtracting the OP-free waveform ([7], [9]) from the electroretinogram ([1]).