To early increase catalytic activity just after an engine is started, subsequently suppress a rise in back pressure and prevent an overrise in an exhaust gas temperature.

When pressure of exhaust gas from an engine is low like just after the engine is started, exhaust gas flow in first and second routes P1 and P2. Exhaust gas flows in only the first route P1 because a valve element 23 remains clogging the downstream end of the second route P2. Because heat insulation is high because this first route P1 is surrounded by the second route P2, lowering of a temperature in exhaust gas flowing in the first route P1 is suppressed as much as possible. When pressure of exhaust gas from the engine is high, exhaust gas flows in the first and second routes P1 and P2 and the valve element 23 opens the downstream end of the second route P2. Thus, a rise back pressure can be suppressed because flowing cross sectional area is increased and an overrise in an exhaust gas temperature can be prevented because exhaust gas flowing in the second route P2 is properly cooled.