PURPOSE: To recognize a desired exposure field even if photographing is executed by using a radiation shielding material in the method for recognizing the radiation exposure field in accordance with the image signal obtd. from a recording sheet on which radiation image information is recorded by applying an exposure field diaphragm.

CONSTITUTION: Edge candidate points are determined in accordance with the image signals along the plural directions included within the exposure field from the image signals obtd. from the recording sheet. On the other hand, the region where the image signals between the respective points exist is removed from the region enclosed by the lines along the edge candidate points if the points turning negative and positive beyond the prescribed value exist from the point where the differential value in the plural directions of the image signals is a prescribed point to the end part of the recording sheet. The region is determined as the radiation exposure field.