The RGB values of each pixel in every frame are converted into ideal tristimulus values X, Y, and Z. Based on a chromaticity-histogram based on the tristimulus values X, Y, and Z and a color gamut for each Y value corrected in accordance with a backlight-brightness, an xy error count generation unit obtains the number of chromaticity errors for each of a plurality of backlight-brightnesses. A lightness-histogram is created based on the RGB signals for each frame. Based on the histogram and a lightness higher than a maximum tone after correction according to the backlight-brightness, a lightness error count generation unit obtains the number of lightness errors for each of the plurality of backlight-brightnesses. An error minimum BL-brightness detection unit decides an optimum backlight-brightness based on the number of chromaticity errors and the number of lightness errors. A tone conversion unit performs tone conversion in accordance with the backlight-brightness.