A first scanner unit and a second scanner unit simultaneously read the front surface and rear surface of a document, respectively, once stores the read image data in a first memory while adjusting the timing by a memory I/F, and then transfers the same to an ASIC. The image data is read out of the first memory and is inputted into an R channel and a B channel at an identical timing. The ASIC executes predetermined image processings and stores monochrome data on the front surface and rear surface of the document in a second memory through a PCI bus in parallel.