An image processing method is intended to extract an irradiation area in an image easily and accurately. A calculation unit 202 calculates secondary difference values of density values representing respective areas in three areas of a rectangular shape arranged in parallel on an image, determined by a calculation area input unit 200 and a calculation area determination unit 201, between adjacent rectangular areas and the secondary difference values thus calculated are stored in a memory unit 203. Then a judgment unit 204 judges one end point of an irradiation area from the secondary difference values thus stored and an irradiation area determination unit 205 determines the irradiation area, based on a plurality of end points thus judged.