A valve seat structure is secured to an outlet end of a passing pipe installed in an exhaust muffler. The valve seat structure includes a flat seat surface portion which extends around the outlet end of the passing pipe. A valve plate structure is pivotally connected to the valve seat structure. The valve plate structure includes a valve plate portion and a flat sealing portion which forms a peripheral part of the valve plate portion. The valve plate structure has a close position wherein the valve plate portion closes the outlet end of the passing pipe having the flat sealing portion entirely pressed against the flat seat surface of the valve seat structure and an open position wherein the valve plate portion opens the outlet end of the passing pipe having the flat sealing portion separated from the flat seat surface. A biasing structure biases the valve plate structure to assume the close position.