A headrest control device for a treatment chair in which the headrest contains a linkage capable of raising and lowering a head supporting portion of the headrest in synchronized interlocking relationship with the tilting of the headrest. The device facilitates the bringing of the head supporting portion into firm contact with the rear neck region of the patient thereby making it possible for him to open his mouth in a natural posture when he undergoes oral treatment with the backrest of the treatment chair being held approximately horizontal and the headrest being slightly forwardly tilted.