In a digital copying machine, a ground in a document image is removed according to the type of document by referring to a histogram of reflectance data on a document image prepared for each line. A ground peak or a gradation level having the largest frequency in the histogram is determined. Further, frequencies are obtained in three windows appropriate for extracting characteristic quantities for ground, background and characters. A window for the ground quantity is determined according to the ground peak, and a ground range for removing the ground is determined by using the ground peak and the three frequencies. Then, the gradation data are corrected according to the ground range, and an image is formed on a sheet of paper according to the corrected gradation data. Thus, a ground can be removed by analyzing the histogram without a prescan.