The viewing device for the interior of a mouth of a patient includes a penetrating ray emitter adapted to take a view of an inner portion located under an outer surface of an organ arranged in the mouth. There is a pair of augmented-reality glasses having an optical glass through which a user of the pair of glasses can see the interior of the mouth and a viewing camera adapted to take an image of what the user sees through the optical glass. A central unit correlates first images corresponding to those taken by the viewing camera with second images corresponding to those taken by the penetrating-ray emitter.