1. A non-transitory storage medium storing a plurality of instructions executable by a computer of a display device comprising an image taking device and a display, the plurality of instructions, when executed by the computer, causing the display device to perform: controlling the display to display a real-space image which is being taken by the image taking device; determining whether the real-space image taken by the image taking device includes a marker; synthesizing the real-space image and a content associated with the determined marker when it is determined that the real-space image includes the marker; controlling the display to display the real-space image synthesized with the content; controlling the display to display a content-display keeping button in a state in which the real-space image synthesized with the content is displayed on the display, the content-display keeping button being configured to, when operated, keep the content displayed on the display; controlling the display to display the real-space image with the synthesized content on the display such that the content is kept displayed on the display, when the content-display keeping button displayed on the display is operated in the state in which the real-space image synthesized with content is displayed on the display and when the real-space image taken by the image taking device includes the marker; controlling the display to display the real-space image with the synthesized content on the display when the content-display keeping button displayed on the display is not operated in the state in which the real-space image synthesized with the content is displayed on the display and when the real-space image taken by the image taking device includes the marker; controlling the display to display the real-space image with the synthesized content on the display such that the content is kept displayed on the display, when the content-display keeping button displayed on the display is operated in the state in which the real-space image synthesized with the content is displayed on the display and when the real-space image taken by the image taking device does not include the marker; and controlling the display to disappear the content on the display while displaying the real-space image when the content-display keeping button displayed on the display is not operated in the state in which the real-space image synthesized with the content is displayed on the display and when the real-space image taken by the image taking device does not include the marker.