In a simultaneous double-sided document reading, this invention controls by software the document reading order to store image data in a single image storage device in the same order as a document page sequence. The copying apparatus has a reader unit, a printer unit, a controller unit and a document supply control unit. The controller unit has a reader memory, an encoder and a decoder. The reader memory temporarily stores at least two pages of uncompressed image data read from the document by the reader unit. The encoder compresses the image data stored in the reader memory. The compressed image memory stores a plurality of pages of image data compressed by the encoder. The decoder analyzes (expands) the compressed image data stored in the compressed image memory and maps it on the page memory.