PURPOSE: To recline the head rest of the dental medical treatment chair on which a patient is seated without giving a discomfort sensation to the patient, and also, to an angle at which a dentist can execute easily a medical treatment.

CONSTITUTION: A movable block 4 is guided so as to be freely movable along a rail 4a provided in the vertical direction in a back rest installing member 1 installed in a back rest, and to a head rest supporting member attached pivotally to the tip of the back rest installing member 1, a head rest installing member 2 in which a head rest 2a is installed so as to be freely slidable. Also, a guide groove 7 for guiding a roller 6 provided in a supporting arm 5 in which one end, and the other end are attached pivotally to the movable block 4, and the head rest installing member 2, respectively is provided in the back rest installing member 1, and moreover, an angle maintaining device 8 of the head rest for stopping a movement of the head rest installing member 2 is provided.