1. An image processing apparatus which comprises: an image sensor including a plurality of pixels in which neighboring pixels define a set of pixels, each of said pixels operative to detect an image density and produce an indication of the image density at said pixel; a calculating means for calculating a sum of the image densities detected by the pixels in each set and for calculating the difference between the sums of the image densities detected by neighboring sets of the pixels, such that said calculating means detects the contour of the image when the absolute value of such difference is greater than a predetermined value; said calculating means comprising adding means for adding said image densities detected by the pixels in each set and for producing the sum thereof; first and second storing means for alternately storing the sums obtained from said adding means; first and second subtracting means for subtracting and alternately obtaining a first difference between the sum stored in said first storing means and a first subsequently obtained sum from the adding means and a second difference between the first subsequently obtained sum stored in said second storing means and a second subsequently obtained sum from the adding means; means for processing said image so as to increase or decrease the density of the image for contour enhancement when the contour is detected by said calculating means; and a digitizing means for digitizing the image density of the image whose contour has been enhanced by the image processing means.