A dental observation device by which, without displaying, in an overlapping manner, a visible light image of an image-capturing site in an oral cavity captured by a 3-D camera and 3-D image information on the image-capturing site in the oral cavity acquired in advance, an invisible portion in the visible light image is easily recognized.An observation system includes a visible light 3-D camera； a relative position calculator； a 3-D information storage storing 3-D image information acquired in advance； an image display displaying, side by side, a 2-D captured image captured by the visible light 3-D camera and 2-D converted image based on the 3-D image information corresponding to the surgical operation target site； and a display direction adjuster displaying, in the image display, the 2-D converted image in a predetermined direction on the basis of the image-capturing direction detected by the relative position calculator.
