In a method of recognizing an irradiation field, image signals at their respective positions on a recording medium carrying a radiation image stored thereon are detected from image information obtained by image read-out. A difference image constituted by difference values between the image signals is created and scanned in one direction to detect sections each intercepted between a difference value not smaller than a threshold value +Th and a difference value not larger than -Th on each scanning line. Or, the difference image is scanned in two or more directions to detect ranges each comprising a group of the intercepted sections on the scanning lines in each of the scanning directions. The total of the sections or a region formed based on the ranges is the irradiation field. In a method of adjusting the image processing conditions, the image processing conditions are adjusted based on the image information inside of the recognized irradiation field.