PURPOSE: To increase the max. output in the high speed high output operation, increasing the max. torque in the intermediate speed high torque operation.

CONSTITUTION: The valve closing spring 17 of a waste gate valve 15 is constituted by arranging a spring 17a for the intermediate speed high torque and a spring 17b for the high speed high output in parallel. The spring 17a for the intermediate speed high torque is elastically deformed at the intermediate speed matching point of the blower efficiency, and the engine revolution speed.supercharge pressure variation curve is deformed to a gentle inclination at the intermediate speed matching point by half opening a valve body 20a. The spring 17b for high speed high output is elastically deformed at the high speed matching point of the blower efficiency, and the engine revolution speed.supercharge pressure variation curve is deformed horizontally at the high speed matching point by perfectly opening the valve body 20a.