In order to provide an optical device for clearly implementing augmented reality provided from a display unit, regardless of a change in a focal point of an outer mirror through which a user views, the present invention comprises: the display unit for emitting an optical beam displaying an image; transparent glass through which external light passes and which forms the path of the optical beam emitted by the display unit; and a pinhole mirror disposed to the glass, and disposed on the path of the optical beam emitted by the display unit, so as to reflect the optical beam emitted by the display unit, wherein the pinhole mirror has a polygonal shape and has an area less than that of the pupil so that an image formed by the optical beam emitted by the display unit overlaps with an image formed by the external light having been transmitted through the glass.