Based on average values for respective color components calculated by an average value calculating section, 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 a number of density bin having a predetermined frequency-of-occurrence value or more for each of the histograms for the maximum value and the maximum difference, and determines that the data of image is of a blank page image when both of the numbers of density bins are not greater than a bin threshold value which is set according to scanning modes or printing modes.