1. An area separating apparatus for judging whether or not each object pixel belongs to a photograph image area in an image on the basis of image data indicative of density gradation levels of pixels constituting the image, the apparatus comprising: an average value calculating circuit for calculating an average value of image data of pixels in a first image block of a predetermined size containing the object pixel; a photograph image area judgment threshold generating circuit for generating a photograph image area judgment threshold in accordance with the average value calculated by the average value calculating circuit; a density difference summing circuit for determining differences in image data between respective adjacent pairs of pixels in the image block and calculating a sum of the image data differences for all the pixels in the image block; and a first judgment circuit which judges that the object pixel belongs to the photograph image area if the sum of the image data differences calculated by the density difference summing circuit is smaller than the photograph image area judgment threshold generated by the photograph image area judgment threshold generating circuit, and judges that the object pixel does not belong to the photograph image area if the sum of the image data differences is not smaller than the photograph image area judgment threshold.