A diagnostic/treatment instrument such as a handpiece equipped with a dental treatment tool, etc. is provided with a light radiating unit that can distinctively detect a dental lesion, to facilitate the treatment of the lesion. A light source for emitting light that can distinctively detect the lesion is mounted near the head of the handpiece to which a diagnostic/treatment tool can be attached. The light emitted from the light source illuminates an area forward of the diagnostic/treatment tool. Alternatively, the light source comprises a plurality of light-emitting devices which are arranged around the periphery of the forward end of the handpiece head in such a manner as to encircle the diagnostic/treatment tool, and radiates light to illuminate the area forward of the diagnostic/treatment tool. When an oral cavity is illuminated with the light, a clinician can observe reflected fluorescent light by using a filtering function that passes the fluorescent light, and thus the clinician can perform the treatment while checking the lesion.
