1. An image displaying apparatus, comprising: an image data reader which reads image data stored in a storage medium; an index image size storage which stores index image sizes predetermined in correspondence with original image sizes of the image data; an index image generator which generates index images of index image sizes corresponding to the original image sizes using the read image data; and an index image displaying device which displays the generated index images on a monitor screen, wherein the index image sizes corresponding to the original image sizes larger than a standard size are stored with larger rates of reduction to the original image sizes in the index image size storage, so that at least one page of the index image is entirely displayed while maintaining a relative dimensional relationship with the index image sizes corresponding to the other original image sizes.