A technique for protecting a screen surface from being damaged due to a fixed display operation of an image display apparatus such as a projection TV. The method includes a first step for displaying an alarm message for informing no external signal input in the mode in which an external signal is inputted, a second step for checking whether an external signal is inputted in the state that the first step is being performed and the mode is changed, and a third step for checking whether the alarm message display time of the first step becomes identical with a set display time and displaying an alarm message when the time exceeds the set time and dynamically displaying a random format graphic or character.