1. An image processing method, comprising the steps of: generating a cumulative histogram of density values of pixels included in at least each of a first image and a second image; calculating a distance between a density value on a first histogram generated from said first image and a density value on a second histogram generated from said second image, in a space defined to include histogram frequencies of the cumulative histograms and the density values; and determining a correspondence relation between the density values included in said first image and the density values included in said second image based on the calculated distances between the density values, and determining a conversion function for correcting a density value between said first image and said second image based on the determined correspondence relation.