A method for adjusting image brightness is conducted by a system for adjusting image brightness. Digital image data acquired by a data acquisition unit are sent to a lightness computing unit and a color saturation computing unit that compute the lightness L and the color saturation C for every pixel and sends the computed values to a mean value computing unit. The mean value computing unit weights the lightness L by the color saturation C and computes weighted mean values Lm of the lightnesses L. A converter 5 converts the R, G, B color signal components constituting the digital image data S to make the weighted mean values Lm desired values.