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, wherein the controller is configured to control the display to display an object requesting one of the first viewing device or the second viewing device to switch the left-eye and right-eye glasses, or to transmit a request to switch the left-eye and right-eye glasses to one of the first viewing device or the second viewing device, and wherein the direction of the displayed object is determined according to the one of the first viewing device or the second viewing device for which the left-eye and right-eye glasses are to be switched.