An image-data loading unit loads image data of an image requested to be displayed based on an input signal in an image memory. A display-image determining unit determines an image to be displayed on the display device based on a priority of the image after the image data is loaded in the image memory. An image-data output unit reads out the image data of the image determined to be displayed by the display-image determining unit from the image memory, and outputs read image data to a display device.