PURPOSE: To reduce the emission of toxic substances with the use of a simple device by adjusting a throttle element in an intake system to an opening position corresponding to a desired value which can be obtained from a characteristic curve relating to a load and a rotational speed so as to correct a desired value of an absolute pressure with respect to the atmospheric pressure.

CONSTITUTION: A throttle valve 4 provided in an intake system 3 is operated by means of a pneumatic actuating element 5 and an electropneumatic pressure transducer 6 which is controlled by an electronic control device 7 in accordance with output signals from an absolute pressure sensor 8, an intake air temperature sensor 10, a coolant temperature sensor 12 and an atmospheric pressure sensor 14. In this arrangement, the throttle valve 4 at each operating point is adjusted to an opening position corresponding to a desired value which is obtained from a characteristic curve relating to a load and a rotational speed for an absolute pressure in the intake system 3 downstream of the throttle valve 4. Further, a desired value of the absolute pressure is corrected with respect to the atmospheric pressure so as to obtain a corrected value which is increased when the atmospheric pressure lowers.