In a supercharged pressure control valve apparatus of an internal combustion engine, a housing of supercharger has first and second by-pass air paths; a valve sheet formed on an end of the second by-pass air path; a rubber diaphragm having thick sealing part of convex and concave sectional shape at its outer periphery; a valve disc fixed to the moving part of the diaphragm and opening closing the first and the second by-pass air paths responding to movement of the diaphragm; and a casing of the valve directly fixed on the housing by sandwiching the outer periphery of the diaphragm.