1. A brightness level adjustment method for adjusting the brightness level of an image, the method comprising the steps of: determining an adjustment amount of a brightness level, which represents the degree of adjustment of the brightness level, for each pixel based on the value of each pixel which forms a luminance distribution image representing the distribution of luminance of the image; determining a portion contributing to brightness, which represents a portion of the adjustment amount of the brightness level, which contributes to adjustment of the brightness, and a portion contributing to saturation, which represents a portion of the adjustment amount of brightness level, which contributes to adjustment of the saturation, for each pixel of the image so that the sum of the portion contributing to brightness and the portion contributing to saturation is equal to the adjustment amount of the brightness level; and adding an adjustment value of saturation, which is obtained by weighting the value of each pixel in the image based on the portion contributing to saturation, and an adjustment value of brightness, which is obtained by weighting the value of each pixel in the luminance distribution image, which corresponds to each pixel of the image, based on the portion contributing to brightness to the value of each pixel which forms the image.