1. A normalization method for photometric measured data in a photographic printing system of the type where frames of original films to be printed are photometric measured in three primary colors as a large number of points thereon, a printing exposure amount is determined based on a large number of photographic measured data obtained from the photometric measurement of said original films, and said original films are printed with said printing exposure amount, said method comprising the steps of: preparing reference data by using either one of the photometric measured data of the three primary colors or average values of at least two photometric measured data of the three primary colors; classifying said reference data on said original films into plural classes in order of value; obtaining first average values of a large number of reference data belonging to respective said classes; obtaining second average values of said photometric measured data for respective said colors by classifying said photometric measured data for respective reference data in accordance with said classes; obtaining a conversion table of said second average values for said first average values; and converting said photometric measured data into said reference data by using said conversion table; whereby photometric measured data of gray images of said original films become a constant color balance.