To provide a backlight unit capable of making variable a color reproduction range of a liquid cristal display device according to a display image and reducing consumption power.

A luminance ratio calculation unit 31 calculates an average value of luminance values of R pixels, an average value of luminance values of G pixels, and an average value of luminance values of B pixels in image data input in one frame, and calculates a first ratio, a second ratio, and a third ratio that are ratios of the average values with respect to a maximum luminance value. A duty ratio calculation unit 32 determines a duty ratio about a red LED, a duty ration about a green LED, and a duty ratio about a blue LED based on those ratios.