A subframe data generator 12 determines a distribution color X which is a color of a variable color subframe, and then selects pixels sequentially to perform the following processing on the selected pixel P. Distributed brightness (Dsr, Dsg, Dsb) is calculated based on brightnesses Dr, Dg, Db and the distribution color X, and a distribution ratio α is set to a value of 1 at which color breakup is the smallest. An evaluation value Qi related to a color difference when a line of sight moves is calculated based on brightness of the selected pixel P, brightnesses of neighboring pixels Pi (i=1 to N), and the distribution color X, and the distribution ratio α is decreased in steps until the maximum value Qmax of the evaluation value Qi is less than or equal to a threshold value Qth. The brightnesses Dr, Dg, Db of three colors are converted to brightnesses Ex, Er, Eg, Eb of four colors based on the distribution color X and the distribution ratio α determined for each pixel. This suppresses irregular flicker that occurs in the vicinity a boundary of pixel areas of different colors.