PURPOSE: To determine optimum readout condition at any time by deciding an optional position where a differential value is larger than a maximum or specific value as the 1st aimed point and a position where a differential value is largest or larger than a specific value among peripheral points adjoining to the 1st aimed point as the 2nd aimed point, and repeating this process and finding new aimed points one after another.

CONSTITUTION: Digital image data at respective positions on a sheet are obtained and a differential image obtained by differentiation is scanned to find an optional point where the differential value is largest or larger than the specific point as the 1st aimed point. The contour of an irradiation field is traced from the 1st aimed point to recognize the irradiation field. This tracing a position where the differential value is larger than the specific value is found as the 2nd aimed point among peripheral positions adjoining to the 1st aimed point and then a point where the differential value is larger than the specific value is found as the 3rd aimed point among peripheral positions adjoining to the 2nd aimed point except said aimed point (1st aimed point). Thus, new aimed points are searched for one after another and when a position adjoining to the 1st aimed point is searched for as a new aimed point, the aimed points obtained so far are connected to draw a closed curve and the area inside of the curve is recognized as the irradiation field.