Uniformize afterimages caused by black insertion to each image when two types of images are alternately displayed. There are provided a left and right image alternate output unit, a mask pattern storage unit, a mask pattern selection counter, and a mask synthesizing unit. The left and right image alternate output unit alternately outputs two types of video frames. The mask pattern storage unit stores m mask patterns, wherein basic regions are defined in a pixel region of a liquid crystal panel, the pixel region has m pixels arrayed in a matrix, m is an even number equal to or greater than 4, the in mask patterns have different arrangements of mask pixels in the basic region, and the number of the mask pixels is an even number smaller than m and equal to or greater than 2.