A method for processing images that reduces an enormous number of images down to images suitable for measuring a similarity index and that measures the similarity index between the reduced number of images more suitably. Image data and attribute information stored in an HDD are read, and an image data unit is selected as a reference on the basis of displaying data according to an image-displaying program. Then, a similarity-measuring program is activated, and the attribute information of the selected image data unit is compared with the attribute information of other image data units. An image data unit to be used for measuring the similarity index is determined, and the similarity index is measured between the selected image data unit and the determined image data units.