1. A method of detecting a base concentration of a film, comprising: a film image entering process for reading a film image by an image pickup device and generating color image data; a concentration histogram generating process for generating concentration histograms for each of RGB color components from the color image data; a shift processing process for shifting concentration histograms of other colors in a concentration axial direction, respectively, with a concentration histogram of a specific color used as a reference; a stretch-processing process for stretching the concentration histograms of the other colors in the concentration axial direction with a minimum concentration value used as a reference after shift processing; a superimposed area calculating process for calculating superimposed area of the concentration histogram, respectively, by combinations of two color components after stretch-processing; a discriminating process for finding a shift rate and a stretch ratio when a total of the respective superimposed areas calculated in said superimposed area calculating process is the greatest value by repeating said shift processing process, said stretch-processing process and said superimposed area calculating process; a base concentration calculating process for specifying a location of a base concentration from the minimum of the lowest concentration in the concentration histograms at the shift rate and the stretch ratio determined in said discriminating process, determining the location of the base concentration of each color component in the original histogram corresponding to the location of the base concentration from the shift rate and the stretch ratio, and calculating the base concentration value in each component of the film from the location of the base concentration; a correction-processing process for correcting RGB components of each relevant pixel of the color image data with the base concentration value calculated in said base concentration calculating process; and an output process for exposing photographic printing paper with an exposure head in accordance with the color image data corrected in said correction-processing process and outputting a photo-print.