1. An image processing apparatus comprising: an input image acquisition module for acquiring an input image generated by reading a document; a histogram generator for generating a histogram of valid pixels in the input image; a character string information calculator for extracting continuous regions in the histogram and calculating character string variance in the input image, based on a distance between a maximum value of classes of the histogram and a center of a continuous region closest to the maximum value, and distances between centers of the continuous regions adjacent to each other; and a document type identification module for identifying a type of the document based on the character string variance.