A controllable exhaust system of an internal combustion engine is shown which includes an exhaust gas inlet tube extending from the engine, a muffler connected at its inlet side to the exhaust gas inlet tube and having first and second exhaust gas flowing passages defined therein, and first and second exhaust gas outlet tubes respectively connected to the first and second exhaust gas flowing passages and extending from the muffler independently. A muffler controller is incorporated with the exhaust system for varying the flow passage area of the second exhaust gas outlet tube in accordance with the pressure of the exhaust gas led into the muffler from the engine. The muffler controller comprises an actuator having a piston rod which is moved straightly and reciprocatively in accordance with the magnitude of the exhaust pressure in the muffler; and a valve assembly including a valve plate, a pivot shaft through which the valve plate is pivotally installed in the second exhaust gas outlet tube, an operation disc coaxially connected to the pivot shaft to rotate together with the pivot shaft and the valve plate, an elongate slot formed in the operation disc and extending radially with respect to the center of the operation disc, and a roller rotatably connected to the piston rod and slidably received in the elongate slot.