PURPOSE: To obtain an output efficiency in a highland similar to the one in a flatland by providing a solenoid valve to be opened and closed by the signals from the height detecting switch on a bypass route.

CONSTITUTION: A height detecting switch 12 is put on in a flatland to open a solenoid valve 11. A part of the supercharged pressure compressed by a compressor 1a of a supercharger 1 is leaked via a suction bypass route 9 to obtain a fixed engine output in a state where a supercharging pressure is lowered. The height detecting switch 12 is put off in a highland to close the solenoid valve 11. This construction permits to increase the supercharging pressure by the compressor 1a of the supercharger 1 to be supplied to an engine main body 7.