A histogram is generated from image data, and two distribution areas are detected. A gamma curve used for gamma correction of the image data is changed into a gamma curve corresponding to the luminance ranges of the two distribution areas. The luminance values of each pixel in the two distribution areas are converted by the gamma correction using the changed gamma curve, and the luminance ranges of the two distribution areas are expanded to the higher luminance side while the relative relation of the luminance the pixels is maintained.