A muffler comprises a casing and an exhaust pipe penetrating the casing. An exhaust
gas chamber 4 is defined between the outer wall surface of the exhaust pipe
and the inner wall surface of the casing. In the exhaust pipe positioned in the
casing, an enlarged chamber for accommodating a particulate filter is formed. A
downstream side portion of the exhaust pipe and the exhaust gas chamber are communicated
with each other by a branch pipe. A communicating port is formed on the wall of
an upstream side portion of the exhaust pipe positioned in the exhaust gas chamber.
Until the pressure difference between the upstream and downstream of the communication
control valve reaches a threshold value, the communication control valve is kept
closed to close the communicating port and, thus, the exhaust gas flows through
the particulate filter. When the pressure difference has reached the threshold
value, the communication control valve is opened by the pressure in the upstream
side portion to open the communicating port and, thus, a part of the exhaust gas
bypasses the particulate filter.
