Abstract: There is disclosed a muffler structure in which a valve opened/closed in accordance with exhaust gas pressure in a pipe is prevented from chattering by an inlet pipe provided with a first side hole and an outlet pipe provided with a second side hole with a valve biased by a spring to simultaneously close both the first and second side holes when the pressure is relatively low. When the pressure of the exhaust gas is relatively high, the pressure exerted on the valve by the exhaust gas in the inlet pipe is sufficient to open the valve and the exhaust gas introduced to the inlet pipe flows out to a second chamber via the first side hole, flows into the outlet pipe via the second side hole, and is exhausted to the outside of the muffler via the downstream outlet opening.