An apparatus for simultaneously reading images on both sides of the document sheet as the sheet having images on both sides is moved through the apparatus, one side read by a CCD sensor and the other side read by a contacted image sensor. An image synthesizing unit synthesizes image data from the CCD sensor and image data from the contacted image within one cycle of main scanning by the two image reading units. Furthermore, image processing circuits in the subsequent steps perform specific modes of processing on the data within one cycle of main scanning. That permits simultaneous reading of both sides of the sheet document without reducing document reading speed and expansion of the image processing unit.