1. An image processing method, comprising: 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 first density value on a first histogram generated from said first image and a second density value on a second histogram generated from said second image, in a space defined to include histogram frequencies and density values with respect to said first and second histograms, wherein said calculating comprises setting weight in accordance with an axial distance corresponding to a histogram frequency in said space, when calculating said distance; determining a correspondence relation between said first density value included in said first image and said second density value included in said second image based on said calculated distance between said first and second density values, and determining a conversion function for correcting a third density value in said first image based on said determined correspondence relation, wherein said determining said correspondence relation comprises determining said correspondence relation between said first and second density values such that a sum of distances between all density values included in said first image and corresponding density values included in said second image is minimized.