In the field of dental care, diagnoses have been and being made by means of X-ray images, visual observation, explorer, palpation, fluorometery, insertion of a pocket probe and observation, observation by means of a root canal meter and the use of a Doppler flood flowmeter. However, such diagnoses are invasive and not very clear and three-dimensional X-ray CT apparatus that are being used for diagnoses are very large and complex. Additionally, OCT apparatus for obtaining optical tomographys by using reflection light that are being used by ophthalmologists give rise to problems because of the profile of the detection probe and operability when such an apparatus is used in an oral cavity that contains hard tissues such as teeth and soft tissues such as gingiva for dental care. The present invention dissolves the above identified problems and provides a non-invasive and high resolution dental optical diagnostic apparatus. A dental optical diagnostic apparatus (1) according to the invention comprises a means (30) for generating guide light to irradiate the dental part of a subject for viewing, visible light for picking up a surface image and signal light of low coherence light for diagnosing the dental part, a camera (69) for picking up a surface image of the dental part, an OCT means for acquiring an optical tomography and an image display section (5) for displaying the obtained images.