A single valve member is provided for selecting communication of one of two flow passages constituting an exhaust flow passage, and continuously providing a flow passage area of the selected one of the two flow passages. According to this valve member, a changeover of flow passage between a main flow passage with an exhaust processing device disposed therein and a bypass flow passage is performed, and each flow passage area is regulated, to meet an output requirement, exhaust processing requirement and muffling requirement, appropriately. For example, a single valve member 5 is disposed on the upstream side or downstream side of a start catalytic converter 3, whereby communication of one of a main flow passage 1 and a bypass flow passage 2 is selected, and a flow passage area of the selected passage is continuously provided.