A histogram of density difference between each pixel and an adjacent pixel is prepared from the image data obtained by pre-scanning an original. Then an approximation function for the density difference histogram is generated, and the kind of the original is discriminated as text/photograph/other based on the coefficient of the approximation function. According to the kind of the original, a density conversion table matching each kind is prepared, and is used for density correction of the image obtained by main scanning. For a text original, the density conversion table is prepared from the distribution of data close to the light and dark ends of the density histogram. For a photograph original, the density conversion table is prepared from the coefficient of the approximation function for the cumulative histogram of the density histogram.