A method, apparatus and program for implementing the method is provided for performing efficient and stable comparative image observation in which a plurality of digital images inputted for comparative observation is switched and displayed on the screen of a display means. Brightness adjusting process for the plurality of images inputted for comparative observation, and aligning process for the structures of the subjects among the images are performed by the image processing means. Display control is implemented by the display control means such that each of the images is repeatedly switched and displayed on the screen of the display means at a favorable frame rate for comparative observation in the range from 1 to 5 fps based on the processed image data files.