To allow an operator to get close to the oral cavity of a patient as possible in order to perform a more accurate dental treatment by narrowing the width of a shoulder section of a backrest of a dental treatment chair.

The dental treatment chair has the backrest 12 and a contour seat 13. The backrest 12 can be erected and flattened with respect to the contour seat 13. The backrest 12 has backrest supports BR and BL extending to both sides for supporting the upper arms of a patient. When performing a dental treatment on a lying patient by flattening the backrest 12, the arms of the patient are supported by the backrests BR and BL. Because the width W of the shoulder section of the backrest 12 can be narrowed in such a configuration, the operator can get more close to the oral cavity of the patient.