PURPOSE: To realize optimum operation in response to operating fuel, by installing plural supercharging pressure setting devices which set maximum supercharging pressure in corresponding to an octane value of the operating fuel, while controlling the supercharging pressure according to a compared result between the properly selected supercharging pressure and the actual supercharging one.

CONSTITUTION: In this supercharging pressure controller 10 which controls pressure at the discharge side, namely, supercharging pressure in a compressor 5 of a supercharger, when the supercharging pressure goes up beyond the setting value, a diaphragm D of an actuator 10c is moved to go to the left against force of a spring S. And, the controller 10 opens a waste gate valve 10a installed in a passage bypassing an exhaust turbine 3 via a rod R and thereby lowers the supercharging pressure down to some extent. In this case, setting supercharging pressure based on a discrimination signal out of a fuel discrimination switch (a setting supercharging selection device) is compared with the actual supercharging one by a suction pressure sensor 7, and when the actual supercharging pressure exceeds the setting one, the opening of a proportional solenoid valve 10b is controlled as well as the amount of supercharging pressure leakage is controlled whereby the maximum supercharging pressure is made so as to be altered.