In a method of adjusting image processing conditions for use in processing of final read-out image signals obtained by final read-out carried out for detecting the image signals used in reproduction of a visible image from a recording medium carrying a radiation image recorded thereon by limitation of an irradiation field, a histogram of the final read-out image signals obtained by the final read-out is created. Prospective edge point signals which are considered to be signals representing edge portions of the irradiation field are detected from the final read-out image signals. A region of a lever lower than the level of a characteristic value representing the prospective edge point signals is removed from the histogram, and the image processing conditions are adjusted on the basis of the histogram outside of the lower level region.