A head-up display (HUD) is known as e.g. a vehicle mounted display for projecting display information on the windshield so that the external information and the display information are simultaneously viewed. In a binocular HUD, the display image is perceived as a double image, and hence is difficult to see. There is a technique for presenting the display image only to one eye for the purpose of preventing the double image in binocular viewing. There is also a technique for presenting the display image only to one eye for the purpose of causing the depth position of the display image to appear farther than its optical display position.
On the other hand, a head mounted display (HMD) is known. The display unit of the HMD is mounted on the user's head. In a monocular HMD, the depth of the display image is unclear.