According to one embodiment, an optical device includes emission of an image projecting light source and emission of an optical path length measuring light source are provided by a light projecting device and respective beams are directed to a moveable mirror. In accordance with the movement of moveable mirror, an image projecting light scans a target and an image is displayed. Reflected light is scattered from the target and reaches a photodetector. By calculating the time difference of reference light and reflected light detected by the photodetector, the distance of the optical device and a reflection point of an optical path length measuring light is computed. By combining the computed optical path length data and angle data of the moveable mirror, the reflecting point of the optical path length measuring light is determined.