1. An image processing apparatus comprising: an input image acquiring module for acquiring a first input image generated by reading one side of a document and a second input image generated by reading the other side of the document; a binarized image generator for generating a first binarized image by binarizing the first input image and a second binarized image by binarizing the second input image; an expanded image generator for generating an expanded image containing an expanded pixel obtained by expanding a valid pixel contained in the first binarized image; and a show-through position identifying module for identifying a show-through position in the second input image, based on the second binarized image and the expanded image.