A method for displaying a shadow of a 3D virtual object, includes steps of: (a) acquiring information on a viewpoint of a user looking at a 3D virtual object displayed in a specific location in 3D space by a wall display device; (b) determining a location and a shape of a shadow of the 3D virtual object to be displayed by referring to information on the viewpoint of the user and the information on a shape of the 3D virtual object; and (c) allowing the shadow of the 3D virtual object to be displayed by at least one of the wall display device and a floor display device by referring to the determined location and the determined shape of the shadow of the 3D virtual object. Accordingly, the user is allowed to feel the accurate sense of depth or distance regarding the 3D virtual object.