When a target image with an aspect ratio of 4:3 is displayed on the screen A of the PDP26 with an aspect ratio of 16:9, the rectangular black images on the left and right of the target image are not solid black. Instead, the luminance of the rectangular black images is lowered stepwise as it becomes more distant from the boundary of the target image, and finally reduced to substantially zero, or to a black image at a position away from the boundary to some extent. This prevents the luminance from sharply falling from the boundary and edge from causing due to the addition of the rectangular black image, thereby preventing burning on a part of the screen A corresponding to the position of the boundary.