To provide a device for identifying caries, plaque, bacterial infection, calculi, dental calculi, and other fluorescent materials on teeth.

The device comprises a means 2 for generating stimulation light A guided to an examined teeth-tissue region, a detecting means 7 and evaluating means for detecting fluorescence F generated in the irradiated teeth- tissue region in response to the irradiation and evaluating the fluorescence, and a beam splitter disposed in a light passage between the means 2 and the examined teeth-tissue region and formed substantially of the flat plane side 4 on the rear of a hemispherical lens 3. The beam splitter reflects the stimulation light A in the direction of the teeth-tissue region and substantially transmits the fluorescence F. By this method, an optical diagnostic apparatus eviclently smaller than a current one can be provided.