A binarizing section generates binary image data representing a binary image from raster image data representing a raster image. A white-pixel ratio determining section determines, based on the binary image data, a white pixel ratio for each line of the binary image and a position of each of lines having a white pixel ratio equal to or greater than a predetermined first threshold among the lines of the binary image, and also determines a white pixel ratio for an entirety of the binary image. An image-type determining section determines that the raster image is a photographic image when the white pixel ratio of the entirety of the binary image is equal to or less than a predetermined second threshold, and that the raster image includes a text image when the lines having a white pixel ratio equal to or greater than the first threshold appear cyclically in the binary image.