1. A method of recognizing an irradiation field wherein an irradiation field is recognized in a case where a radiation image is recorded on a recording medium by limitation of the irradiation field, the method of recognizing an irradiation field comprising the steps of: (i) extracting sample image signals at an arbitrary picture element string, which extends from an edge of a recording region on said recording medium towards the center of said recording region, from image signals detected from said recording medium, (ii) expressing changes among image density levels at a predetermined number of picture elements in the vicinity of said edge of said recording region, which image density levels are represented by said sample image signals, by an approximate equation substantially constituted by a simple equation, (iii) calculating differences between imaginary image density levels calculated by use of said approximate equation and actual image density levels represented by said sample image signals, and (iv) recognizing a region extending up to a picture element, at which said difference comes up to a predetermined value, from said edge of said recording region towards the center of said recording region as a region outside of the irradiation field, and recognizing a region inward from said picture element as viewed in the direction from said edge of said recording region towards the center of said recording region as the irradiation field.