An apparatus for diagnosing the environmental tissue of teeth, which has an electro-mechanical oscillation converter for converting the electric oscillation output from an electric oscillation output generation circuit into mechanical oscillation, a probe connected to the converter to be brought into contact with a patient's tooth for applying the mechanical oscillation from the converter to the tooth, and patient's responding means for actuation by the patient when he detects the sense threshold of the mechanical oscillation applied through the probe to the patient's tooth. Thus, a dentist can scientifically diagnose the health of the tissue of the patient's teeth as distinguished from the diseased part without visual examination nor palpation by the dentist.