A display apparatus comprising: an image forming device configured to form an image; an optical system configured to provide an output image by combining light containing an outside landscape with the image formed by the image forming device; and a driving device (120, 120a, 120b, 120c, 102d, 120e) configured to adjust a distance between the image forming device and the optical system, wherein the driving device (120, 120a, 120b, 120c) comprises: a fixed frame; a movable frame which faces the fixed frame and is movable; an actuator (20, 70) configured to change a distance between the fixed frame and the movable frame; and a fixing member (30, 40, 50, 60, 80) configured to be operated in a first state while the distance between the fixed frame and the movable frame is changed and to be operated in a second state while the distance between the fixed frame and the movable frame is maintained, such as to fix the distance between the fixed frame and the movable frame at a first distance or at a second distance greater than the first distance; wherein the image forming device is fixed to the movable frame; wherein the actuator comprises: a first elastic bridge having a curved surface which is convex toward the fixed frame; a second elastic bridge having a curved surface which is convex toward the movable frame; a variable length element fixed between both ends of the first elastic bridge and both ends of the second elastic bridge, and respectively having a length that is variable. an image forming device configured to form an image; an optical system configured to provide an output image by combining light containing an outside landscape with the image formed by the image forming device; and a driving device (120, 120a, 120b, 120c, 102d, 120e) configured to adjust a distance between the image forming device and the optical system, wherein the driving device (120, 120a, 120b, 120c) comprises: a fixed frame; a movable frame which faces the fixed frame and is movable; an actuator (20, 70) configured to change a distance between the fixed frame and the movable frame; and a fixing member (30, 40, 50, 60, 80) configured to be operated in a first state while the distance between the fixed frame and the movable frame is changed and to be operated in a second state while the distance between the fixed frame and the movable frame is maintained, such as to fix the distance between the fixed frame and the movable frame at a first distance or at a second distance greater than the first distance; wherein the image forming device is fixed to the movable frame; wherein the actuator comprises: a first elastic bridge having a curved surface which is convex toward the fixed frame; a second elastic bridge having a curved surface which is convex toward the movable frame; a variable length element fixed between both ends of the first elastic bridge and both ends of the second elastic bridge, and respectively having a length that is variable. a fixed frame; a movable frame which faces the fixed frame and is movable; an actuator (20, 70) configured to change a distance between the fixed frame and the movable frame; and a fixing member (30, 40, 50, 60, 80) configured to be operated in a first state while the distance between the fixed frame and the movable frame is changed and to be operated in a second state while the distance between the fixed frame and the movable frame is maintained, such as to fix the distance between the fixed frame and the movable frame at a first distance or at a second distance greater than the first distance; wherein the image forming device is fixed to the movable frame; wherein the actuator comprises: a first elastic bridge having a curved surface which is convex toward the fixed frame; a second elastic bridge having a curved surface which is convex toward the movable frame; a variable length element fixed between both ends of the first elastic bridge and both ends of the second elastic bridge, and respectively having a length that is variable. a first elastic bridge having a curved surface which is convex toward the fixed frame; a second elastic bridge having a curved surface which is convex toward the movable frame; a variable length element fixed between both ends of the first elastic bridge and both ends of the second elastic bridge, and respectively having a length that is variable.