A duplex simultaneous reading apparatus having an image reading unit for simultaneously reading a first surface and a second surface of a document, the duplex simultaneous reading apparatus includes: a detection unit for detecting any specified pattern image from image data read by the image reading unit; and a selection unit for selectively inputting the read image data of the first surface of the document and the read image data of the second surface to the detection unit in block units, wherein the detection unit detects the specified pattern image in the first surface or the second surface of the document, from the block-unit image data inputted by the selection unit.