An image processing apparatus including two image reading units can sequentially execute image processing of front face image data and back face image data of a document using a single image processing circuit. The image processing apparatus transfers the first face image data of the read document to a first ring buffer and the second face image data thereof to a second ring buffer, respectively, while switching, predetermined unit by unit, whether to transfer the first face image data or the second face image data of the image data to an image processing unit.