13. A computer-readable, non-transitory medium storing a computer program, wherein said computer program causes a computer to execute a process, the process comprising: acquiring an input image generated by reading a document; generating a histogram of valid pixels in the image; extracting continuous regions in the histogram and calculating a 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 identifying, using a computer, a type of the document based on the character string variance.
13. A computer-readable, non-transitory medium storing a computer program, wherein said computer program causes a computer to execute a process, the process comprising: acquiring an input image generated by reading a document; generating a histogram of valid pixels in the image; extracting continuous regions in the histogram and calculating a 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 identifying, using a computer, a type of the document based on the character string variance. 16. The computer-readable, non-transitory medium according to claim 13 , the process further comprising identifying whether the document has a monochromatic print side, wherein the type of the document is identified further based on whether the document has the monochromatic print side.