Abstract: An exhaust gas control valve comprising a diaphragm interposed between an outer upper casing and an outer lower casing of the diaphragm housing, the diaphragm and the upper outer casing defining a first pressure chamber, an inner upper casing attached to the upper surface of the diaphragm within the first pressure chamber, an inner lower casing attached to the undersurface of the diaphragm within the outer lower casing, the inner lower casing and the diaphragm defining a second pressure chamber, a valve housing attached to the bottom of the diaphragm housing and provided with an exhaust gas intake port, an exhaust gas discharge port and a valve seat member disposed in a passage between the exhaust gas intake and discharge ports, a valve head with a stem whose upper end is attached to the center of the diaphragm so that the valve head may be moved toward and away from the valve seat member depending upon the deflection of the diaphragm, the space or chamber defined by the diaphragm and the inner upper casing a