PURPOSE: To permit proper control of air fuel ratio over the whole range of running in an internal combustion engine with a supercharger by detecting the supercharging condition through a plurality of pressure switches different from each other in set pressure to increase stepwise fuel in high supercharging.

CONSTITUTION: A turbine 17 and a compressor 19 are provided in an internal combustion engine 1. A plurality of pressure switches 6aW6c operated under different set pressure are provided in an intake path downstream of a throttle valve 4 while an oxygen sensor 3 is provided in an exhaust manifold 2. A control unit 8 judges stepwise the supercharging condition through pressure switches 6aW6c to displace a control apparatus 32 and supply fuel flow suited for the supercharging condition. Thus, proper air fuel ratio can be controlled over the whole range of running.