1. A photograph image-processing method, comprising the steps of: entering a film image for reading a film image by an image pickup device, generating a plurality of pieces of color image data and storing the generated color image data in a memory; generating concentration histograms for each of RGB color components from all of the color image data of one roll of a film which are stored in the memory; relative-stretch-processing the concentration histograms in a concentration axial direction; calculating and deriving a relative stretch ratio which maximizes a superimposed area of each RGB concentration histogram 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; conversion processing respective RGB components of each pixel of the color image data in accordance with the correction stretch ratio; and exposing photographic printing paper with an exposure head in accordance with the color image data of which RGB components of each pixel are conversion processed to output a photo-print.