An average value calculating section calculates, for respective color components, an average value of pixel values within a pixel block, to define the calculated average values as a pixel value of a pixel of interest. A histogram generating section obtains, for respective pixels of interest, a maximum value of the average values among the color components and a maximum difference in the average values among the color components, and generates histograms respectively for the maximum value and the maximum difference in which histograms a number of pixels for a density bin is considered as a frequency-of-occurrence value. A determination section obtains numbers of density bins that correspond to a frequency-of-occurrence value greater than or equal to a predetermined frequency-of-occurrence value in the histograms, and determines that data of image is of a blank page image when both of the numbers of density bins are a bin threshold or less.