Provided is a muffler comprising: an outer shell; an outlet pipe, which is formed into a tubular shape containing a pair of open end-parts and is comprising an inlet port, wherein one of the open end-parts and the inlet port are arranged inside the outer shell; a valve body formed into a plate-like shape to cover the inlet port from the inside of the outlet pipe; an axis member that pivotally supports the valve body to the outlet pipe so that the valve body pivots inside the outlet pipe; and a biasing member that applies a biasing force in the radial direction of the outlet pipe to the valve body so as to occlude the inlet port.