To provide an exhaust system for an internal combustion engine, reducing its weight and also reducing production cost, by providing an on-off valve requiring no complicated control and having a simple structure so that an increase in sound pressure is suppressed by the air column resonance of a tail pipe.

A shutter member 41 swinging around a shaft member 42 is provided so that only an exhaust flow is received and the passage cross-sectional area of the downstream opening end 28b of the tail pipe 28 is varied. When the flow rate of the exhaust flow is less than a predetermined flow rate, the shutter member 41 is positioned in the node of the sound pressure distribution of the standing wave of the air column resonance generated in the tail pipe 28 so that the passage cross-sectional area is reduced. When the flow rate of the exhaust flow is the predetermined flow rate or more, the shutter member 41 is swung by a predetermined angle from a position as the node of the sound pressure distribution of the standing wave of the air column resonance so that the passage cross-sectional area of the downstream opening end 28b is enlarged in comparison with the case that the shutter member 41 is in a closing position.