An image display device, a 3D viewing device, and a method for operating the same are provided. The method includes receiving a synchronization signal from an image display device, alternately opening a left glass and a right glass according to the synchronization signal, and simultaneously opening the left glass and the right glass in a left-eye image display period or a right-eye image display period of a 3D image displayed on the image display device when a period of the synchronization signal has changed. This ensures that a user reliably (or correctly) views 3D or 2D images using the 3D viewing device.