1. An image processing method of making luminance correction on the basis of a luminance histogram showing distribution of a luminance level of image data, comprising the steps of: obtaining a luminance average value in said luminance histogram and a peak distance showing a distance between peaks in said luminance histogram; comparing a distribution discrimination value which can discriminate whether a distribution deviation of the luminance level exists on a low luminance side or a high luminance side in said luminance histogram or not with said distance between the peaks, thereby discriminating whether said image data is data of a backlight image or not on the basis of a result of said comparison; and comparing said luminance average value with an exposing state discrimination value which can discriminate the exposing state, thereby discriminating whether said image data is data of an image other than the backlight image or not.