PURPOSE: To reduce the number of component parts and the installation space to half, by employing such an arrangement that restriction of the vehicle speed and prevention of abnormal rise of the supercharging pressure caused when a waste gate valve is damaged are both achieved by a single relief valve.

CONSTITUTION: When the vehicle speed is lower than a prescribed value, a vehicle-speed switch 31 is turned ON and a solenoid valve 27 is not energized, so that passages 26 and 28 are communicated with each other. When supercharging pressure is introduced into a diaphragm chamber 23 of a relief valve 15, a valve body 19 is closed since the load of a spring 20 is set at a value higher than the maximum value of normal supercharging pressure. Here, if a waste gate valve becomes inoperable due to damage and supercharging pressure is raised, the valve body 19 is opened. Further, when the vehicle speed reaches a prescribed value, the vehicle-speed switch 31 is turned ON. At the same time, the diaphragm chamber 23 of the relief valve 15 is released to the atmosphere via the solenoid valve 27 and the load of the spring 20 is set at a value lower than the supercharging pressure, so that the valve body 19 is opened immediately for releasing pressurized air.