Abstract: A muffler assembly, for an exhaust system of an internal combustion engine has an exhaust gas inlet tube projected into the muffler to feed the muffler with the exhaust gas of the engine. First and second flow passages are defined through which the exhaust gas from the muffler flows independently before discharged to the open air. The controller has a pneumatic actuator that has a piston rod and a work chamber. The piston rod is adapted to reciprocate linearly in accordance with the magnitude of pressure developed in the work chamber. The pressure of the exhaust gas of the engine is fed to the work chamber of the pneumatic actuator. The actuator controls a valve assembly which includes a valve plate and a pivot shaft through which the valve plate is pivotally installed in the second flow passage of the first means.