A display mode switching device and method automatically switches a display mode of a television set to an overscan mode or to an underscan mode depending on a content of a screen. In a case of connecting a television set to a computer, a call from an application for an API in an OS is detected by a device driver when a user starts playing a DVD or a game. Accordingly, an output mode for the television set is switched to the overscan mode. In this way, a motion video or a game is displayed on the television set in the overscan mode. Meanwhile, when the user finishes playing the DVD or the game, a call for the API to release resources is detected by the device driver, and the output mode is restored to the underscan mode. In this way, a computer screen is displayed on the television set in the underscan mode.