Luminance levels of an analog or digital image signal entered from outside are detected as a luminance histogram. Based on the luminance histogram, a black luminance level and a white luminance level are determined, and are assigned respectively to a darkest tonal level and a lightest tonal level of a gradation range available for printing. To those pixels having luminance levels between the black and white levels, tonal levels between the darkest and lightest tonal levels are assigned according to a gradation conversion curve. The gradation conversion curve is selected among from a plurality of predetermined conversion curves depending upon a scene discrimination coefficient. The scene discrimination coefficient is calculated by multiplying the frequency of each luminance level shown in the luminance histogram with a different luminance coefficient predetermined according to the luminance level, and accumulating multiplication results of all the luminance levels.