The invention provides an image processing apparatus which can correct distortion of read image data using only the image data without using a two-dimensional sensor. The image processing apparatus includes an estimation section for estimating a displacement amount between part data which form image data based on a cross-correlation between the part data, and a correction section for correcting the image data so as to cancel a displacement in the image data based on the displacement amount estimated by the estimation section. The image processing apparatus is used to correct distortion of an image, particularly a document image, inputted from an image inputting apparatus such as, for example, a handheld image scanner.