1. A method of detecting the 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 said color image data; a shift processing process for shifting concentration histograms of other colors in the 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 other colors in the concentration axial direction with the 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 the shift rate and the stretch ratio when a total of the respective superimposed areas calculated is the greatest value; and a base concentration calculating process of specifying the 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 histogram from said shift rate and said stretch ratio, and calculating the base concentration value in each component of the film from the location of the base concentration.