A black line inserting section (3) inserts first grayscale inserts first grayscale lines, each of which has a color of certain grayscale and has a width corresponding to a certain number of pixels, into each of first video images such that a certain distance is secured, between the respective first grayscale lines, in a vertical or horizontal direction of each of the first video images. The black line inserting section (3) also inserts second grayscale lines into each of second video images, which corresponds to a certain number of frames and by which a corresponding one of the first video images is followed, such that the second grayscale lines are not inserted into second parts corresponding to first parts where the respective first grayscale lines are inserted.