1. An image discrimination apparatus, which extracts the data for a feature from an autoradiograph that is produced when a subject is irradiated with radioactive rays by a radioactive device and which employs the obtained data for the feature to identify the category of said subject in said autoradiograph, comprising: (A) area extraction means for entering an autoradiographic signal, for detecting a plain image area indicated by said autoradiographic signal, and for extracting an area other than said plain image area that is detected and an area, which is in contact with said plain image area, having a predetermined width; (B) maximum density value calculation means for calculating the maximum density value for one pixel of those pixels that constitute said area, and the coordinates of the location of said pixel in said autoradiograph; (C) feature data calculation means for calculating the data for the feature of said autoradiograph by employing said maximum density value for said pixel and said coordinates for said pixel in said autoradiograph, both of which are obtained by said maximum density value calculation means; and (D) discrimination means for employing said data for said feature obtained by said feature data calculation means to identify the category of said subject in said autoradiograph.