1. An image processing apparatus comprising: a scanning part operable to scan opposing pages of a book that is laid-open facing-down against a scan plane and to generate input image data based thereon; an image processing part operable to determine a plurality of candidate pixels having variation of brightness exceeding a first reference value in a first direction, among a plurality of pixels constituting the input image data, to determine a folding area region of the input image data representing a central portion of the opposing pages of the book based on a number of the candidate pixels existing in a row of pixels in a second direction different from the first direction, and to generate output image data by adjusting pixel values of the input image data in the determined folding area region; and a printing part operable to print the output image data on a record medium.