Technology for controlling image display according to the state of a viewer wearing eyeglasses to view 3D images is provided. When an inclined angle of eyeglasses is in a predetermined range, an image display device alternately displays a left-eye image and a right-eye image. When the inclined angle of the eyeglasses exceeds the predetermined range, the image display device does not switch the left-eye image with the right-eye image. For example, the image display device displays 2D images. Optical shutters of the eyeglasses are synchronized with the left-eye image and the right-eye image. When the image display device does not switch the left-eye image with the right-eye image, the optical shutters are both open.