PURPOSE: To permit supercharged pressure to be arbitrarily adjusted by a driver, by interposing a solenoid valve on the way of a piping connecting an output side of a compressor section with a supercharged pressure detecting section, and controlling an opening and closing operation of the solenoid valve according to a result of comparison of a reference value as arbitrarily determined with a detection value of a pressure sensor.

CONSTITUTION: A supercharged pressure setting volume 5 is operated by a driver to determine a desired value of supercharged pressure. A control section 3 acts to open a solenoid valve 1a and close a solenoid valve 1b under a normal running condition. When an actual supercharged pressure becomes lower than the predetermined supercharged pressure, the control section 3 operates to compare an output voltage of a pressure sensor 2 with a reference voltage to increase an opening time of the solenoid valve 1b. On the contrary, when the actual pressure is higher than the predetermined pressure, the control section 3 operates to decrease the opening time of the solenoid valve 1b. When the supercharged pressure is increased, difference between the actual supercharged pressure and the predetermined supercharged pressure is rendered small, and the opening time of the solenoid valve 1b is shortened. Accordingly, pressure to be applied to a supercharged pressure detecting section D is increased, and at the time when the difference between the actual supercharged pressure and the predetermined supercharged pressure becomes zero, the supercharged pressure detecting section D functions to open a bypass B.