A computer-readable medium is encoded with a computer program for operating a computer to control a radiation detector and a display apparatus. The computer is operated to derive a processed image by subtraction processing of a first image derived by a first energy radiation having a first energy and transmitting through an object and a second image derived successively to the first image by a second energy radiation having a second energy different from the first energy and transmitting through the same object, among a plurality of images of successive frames detected by the radiation detector detecting as an electric signal the radiation transmitted through the object. The display is operated to display on the display apparatus successively a plurality of the processed images derived thereby.