PURPOSE: To secure good valve operation by forming a waste gate valve for opening and closing a turbine outlet having superior strongness and circular shape and also forming the opening portion of an exhaust passage connected to the turbine outlet long oval housed in the circle of the valve.

CONSTITUTION: A turbine case 1 is divided into three scroll chambers 1A-1C in the rotary direction of a turbine impeller 2 by partition walls 3. The respective scroll chamber 1A-1C are connected to a turbine outlet via respective exhaust bypass 4A-4C. The turbine outlet is opened and closed by a waste gate valve 5. In this case, the waste gate valve 5 is formed circularly and superior in strongness and is rotatably fitted to a valve stay 6. The opening portions of the respective exhaust bypass passage 4A-4C are formed long oval to be housed in the circularness of the waste gate valve. Accordingly, good valve operation is secured.