To reduce the discharge quantity of harmful components in exhaust gas immediately after the start of an engine.

An-exhaust damper 18 for opening/closing an exhaust pipe 16 is provided downstream of a catalyst 17, and this exhaust damper 18 is driven to open/close by a pneumatic actuators 21. A spring 24 is enclosed in a pressure chamber 23 of the pneumatic actuator 21, and the exhaust damper 18 is opened/closed by the grade relation between air pressure P3 and the spring force of the spring 24. The pressure chamber 23 is communicated with the atmosphere through a micro throttle 25. After the stop of an engine, intake negative pressure is led into the pressure chamber 23 to close the exhaust damper 18. The catalyst 17 is thereby insulated thermally so as to heighten exhaust emission performance at the start time after that. When the air pressure P in the pressure chamber 23 becomes smaller than the spring force of the spring 24 due to micro pressure leakage from the micro throttle 25 during the closure of the exhaust damper 18, the exhaust damper 18 is opened by the spring force of the spring 24.