A head mounted display device which is wearable by an observer, comprising: a display unit configured to display an image so as to be capable of being visually recognized by the observer; and a main body having the display unit provided therein, wherein the main body is mountable on a head of the observer, wherein the main body includes a plurality of cases (21, 232), and the plurality of cases includes an exterior case constituting an exterior, and an inside case; wherein the exterior case includes a space for housing the inside case, the inside case being arranged within the exterior case so that at least a portion of the inside case is covered with the exterior case; wherein the inside case has a control device housed therein, the control device for controlling at least a portion of a function of the head mounted display device; wherein the inside case has a waterproof structure and includes a box-shaped case main body having a U-shaped section in which an internal space for housing the control device is formed, and which includes an opening for housing the control device in the internal space, and a cover member that covers the opening, and is attached to the case main body by an adhesive; wherein the exterior case includes a first curved surface along a mounting region of the head mounted display device for facing the forehead and temporal region of the observer; wherein the case main body includes a second curved surface that faces the first curved surface and is curved along the first curved surface; wherein the opening is located on the second curved surface; and wherein the cover member is attached to the second curved surface. a display unit configured to display an image so as to be capable of being visually recognized by the observer; and a main body having the display unit provided therein, wherein the main body is mountable on a head of the observer, wherein the main body includes a plurality of cases (21, 232), and the plurality of cases includes an exterior case constituting an exterior, and an inside case; an exterior case constituting an exterior, and an inside case; wherein the exterior case includes a space for housing the inside case, the inside case being arranged within the exterior case so that at least a portion of the inside case is covered with the exterior case; wherein the inside case has a control device housed therein, the control device for controlling at least a portion of a function of the head mounted display device; wherein the inside case has a waterproof structure and includes a box-shaped case main body having a U-shaped section in which an internal space for housing the control device is formed, and which includes an opening for housing the control device in the internal space, and a cover member that covers the opening, and is attached to the case main body by an adhesive; a box-shaped case main body having a U-shaped section in which an internal space for housing the control device is formed, and which includes an opening for housing the control device in the internal space, and a cover member that covers the opening, and is attached to the case main body by an adhesive; wherein the exterior case includes a first curved surface along a mounting region of the head mounted display device for facing the forehead and temporal region of the observer; wherein the case main body includes a second curved surface that faces the first curved surface and is curved along the first curved surface; wherein the opening is located on the second curved surface; and wherein the cover member is attached to the second curved surface.