1. An image display apparatus comprising: a display to display a 3D image having a left-eye image and a right-eye image; a sensor to detect positions of a first viewing device and a second viewing device for viewing the 3D image; and a controller configured to request that a left-eye glass and a right-eye glass of one of the first viewing device or the second viewing device be switched if the detected positions of the first viewing device and the second viewing device correspond to opposite sides of the display.