To enable a necessary image data to be easily found out based on image sizes of a plurality of index images displayed in a list format and to enable contents of original images displayed as index images to be easily understood, in the case of generating an index image of an image data having an original image size larger than a standard size, a rate of reduction to the original image is increased so that the entire index image is accommodated in an index image display area while maintaining a relative dimensional relationship with index image sizes of image data having other original image sizes.