1. An image joining apparatus that reads a single document in a plurality of parts and joins a plurality of read images together, the image joining apparatus comprising: a reading unit configured to read a document; a memory configured to store images read by the reading unit; a first storage control unit configured to compress and store, in the memory, the images read by the reading unit; and a second storage control unit configured to store, in the memory, joint images with a compression ratio lower than that with which the images read by the reading unit are compressed by the first storage control unit, the joint images being end portions of the respective read images, and the end portions being portions to be joined together, wherein the images stored by the first storage control unit are joined together at relative positions determined based on the joint images stored by the second storage control unit.