PURPOSE: To adapt a supercharging pressure in an intake pipe to the characteristics of an internal combustion engine by a method wherein when a differential pressure between a detecting supercharging pressure in the intake pipe and a target supercharging pressure is a positive pressure, a pressure in the intake pipe is reduced by an amount equivalent to a differential pressure through utilization of a control air pressure corresponding to the differential pressure.

CONSTITUTION: Pressure air in an intake pipe 22 for an internal combustion engine 20 is accumulated at a set pressure in a pressure accumulating tank 41. When a switching valve device 46 is communicated, the pressure is regulated by a regulator 44 to a control air pressure, which is applied on the pressure chamber 33 of an actuator 30. In this case, the rotation speed of the internal combustion engine 20 and a supercharging pressure by a supercharger 1 in the intake pipe 22 is detected by sensors 51 and 54. In which case, a detecting supercharging pressure is compared with a target supercharging pressure by an electric control circuit 50 to calculate a differential pressure therebetween. As a result, when the differential pressure is a negative value, the pressure chamber 33 is communicated with outside air. Meanwhile, when the differential pressure is a positive value, a control air pressure corresponding to a differential pressure is applied on the pressure chamber 33 to open a valve 12, and a pressure in the intake pipe 22 is reduced by an amount equivalent to the differential pressure.