A sealing device for an exhaust manifold includes a main pipe having a connection portion including a mounting groove formed at an interior circumferential surface; a sub pipe having a protruding portion at one end portion, the sub pipe being coupled to the main pipe in a state in which the protruding portion is inserted into the connection portion; a valve mounted at the mounting groove within the connection portion; and a gasket interposed between the protruding portion and the valve within the connection portion.