A display apparatus, comprising: a frame that is configured to be worn on a head part of an observer; an image display apparatus (100, 200, 300, 400, 500) provided in the frame; and a control apparatus, wherein the image display apparatus (100, 200, 300, 400, 500) includes an image forming device (111, 211); an optical device (120, 320, 520) configured to allow light emitted from the image forming device (111, 211) to be incident thereto, to be guided, and to be emitted therefrom; and a dimming device configured to adjust a light amount of ambient light incident from an outside, the dimming device and the optical device (120, 320, 520) are preferably provided in this order from the outside, the light amount of ambient light incident from the outside is reduced by the dimming device, the display apparatus further comprises an imaging apparatus; and the image display apparatus (100, 200, 300, 400, 500) is configured to display, in a first mode, an image of a portion of an environment viewable through the image display apparatus captured by the imaging apparatus, characterized in that the display apparatus further includes an image information storage device or a communication unit configured to receive data from an external image information storage device, wherein, the display apparatus is configured to compare data regarding a subject imaged by the imaging apparatus with information that is stored in the image information storage device or with information obtained via the communication unit from the external image information storage device, the image display apparatus (100, 200, 300, 400, 500) is configured to display in the first mode an image of the subject conforming to the information stored in the image information storage device or conforming to the information obtained from the external image information storage device in a state where a light amount of ambient light incident from the outside is reduced by the dimming device; and the image display apparatus (100, 200, 300, 400, 500) is configured to display, in a third mode, an image of the subject conforming to the information stored in the image information storage device or conforming to the information obtained from the external image information storage device in a state where a light amount of ambient light incident from the outside is not reduced by the dimming device. a frame that is configured to be worn on a head part of an observer; an image display apparatus (100, 200, 300, 400, 500) provided in the frame; and a control apparatus, wherein the image display apparatus (100, 200, 300, 400, 500) includes an image forming device (111, 211); an optical device (120, 320, 520) configured to allow light emitted from the image forming device (111, 211) to be incident thereto, to be guided, and to be emitted therefrom; and a dimming device configured to adjust a light amount of ambient light incident from an outside, an image forming device (111, 211); an optical device (120, 320, 520) configured to allow light emitted from the image forming device (111, 211) to be incident thereto, to be guided, and to be emitted therefrom; and a dimming device configured to adjust a light amount of ambient light incident from an outside, the dimming device and the optical device (120, 320, 520) are preferably provided in this order from the outside, the light amount of ambient light incident from the outside is reduced by the dimming device, the display apparatus further comprises an imaging apparatus; and the image display apparatus (100, 200, 300, 400, 500) is configured to display, in a first mode, an image of a portion of an environment viewable through the image display apparatus captured by the imaging apparatus, characterized in that the display apparatus further includes an image information storage device or a communication unit configured to receive data from an external image information storage device, wherein, the display apparatus is configured to compare data regarding a subject imaged by the imaging apparatus with information that is stored in the image information storage device or with information obtained via the communication unit from the external image information storage device, the image display apparatus (100, 200, 300, 400, 500) is configured to display in the first mode an image of the subject conforming to the information stored in the image information storage device or conforming to the information obtained from the external image information storage device in a state where a light amount of ambient light incident from the outside is reduced by the dimming device; and the image display apparatus (100, 200, 300, 400, 500) is configured to display, in a third mode, an image of the subject conforming to the information stored in the image information storage device or conforming to the information obtained from the external image information storage device in a state where a light amount of ambient light incident from the outside is not reduced by the dimming device.