1. An image combining device, comprising: a read unit, which includes: a transparent holder configured to hold a flat medium having a plurality of matching reference points marked in an edge region; and an image read section of flatbed type, configured to read said medium held in said transparent holder, through a reading window, a size of said medium being larger than a size of said reading window, and to generate a plurality of partial images, each of which includes an image of a part of said medium and a part of the plurality of matching reference points, the matching reference points being arranged along a part of an edge of the image of the part of said medium; and a control unit, configured to combine said plurality of partial images based on the matching reference points included in said plurality of partial images, and to generate a combined image including an image of said medium, wherein: said control unit comprises an image compressing section, configured to compress said plurality of partial images to generate compressed images, each of which includes compressed data obtained by compressing a group of data arranged in a first direction of said partial image, decompress a part of said compressed data of each of said compressed images, and combine decompressed data, obtained by decompressing a part of the compressed data, to generate the combined image, and the plurality of partial images arranged in said first direction are combined to generate said combined image.