This paper describes the development of the mouth-opening and closing training robot WY-1 (Waseda Yamanashi-1) that is designed and developed by combining the oral-maxillofacial surgery and the robotics. Mouth-opening training is indicated for the rehabilitation of the patients suffering from disturbance of mouth-opening and closing. Conventional mouth-opening apparatuses such as the wooden screws, bite blocks, and clothespin types increase only mouth-opening distance and have simple mechanisms without actuation, sensor, and/or control system. Moreover, the quantitative force data during therapy are not known in spite of the importance of standardization of the mouth-opening and closing training.<br>The mouth-opening and closing training robot WY-1 developed in 1995 includes four systems: mechanical system, actuation system, sensor system, and control system. Mechanical system includes wooden device that has been usually used in dentistry. Actuation system has a DC motor and a tendon to manipulate the mechanical system's wooden device. Sensor system includes three types of sensors: force, angle, and angular velocity sensors. The strain gage type force sensor attached to the mechanical system's wooden device directly measures the mouth-opening ability. An angle and an angular velocity sensor, that is, an encoder and a tachogenerator, measure the angle and angular velocity of the actuation system's DC motor during robotic therapy. Control system integrates, records, and controls the whole system of the mouth-opening and closing training. Three safety systems, that is, emergency switch to turn off the actuation system, mechanical stopper on the mechanical system's wooden device, and threshold set on the sensor system's output of the force sensor, protect the patient from severe situation.<br>As the result of therapy for the fibrous ankylotic TMJ patient by WY-1, the mouth opening distance increased from 14mm to 21mm. The effectiveness of the robot was confirmed by this therapy.