1. A photograph image-processing method, comprising steps of: entering a film image for reading a film image by an image pickup device and generating color image data; generating concentration histograms for each of RGB color components from the color image data; relative-stretch-processing the concentration histograms; calculating and deriving a relative stretch ratio which maximizes the RGB superimposed area which has undergone relative-stretch processing; calculating and deriving an evaluation value of reliability with respect to the relative stretch ratio; calculating and deriving a correction stretch ratio based on the evaluation value; and conversion-processing RGB components of each pixel of the color image data in accordance with the correction stretch ratio.