PURPOSE: To adjust the ratio of air to fuel in high altitude by changing the supply of secondary air in response to the atmospheric pressure and enging temperature, in a system for supplying secondary air to the inlet system of the internal combustion engine.
CONSTITUTION: Secondary air paths 3, 4 are in communication with an inlet system at the downstream of a benturi 2. These paths 3, 4 are provided with an orifice device 5 which opening is controlled by a control circuit 6, and an air valve 8 to which the negative pressure within a negative pump 9 is transmitted via a surge tank 10. The device 5 comprises a circuit 6 to which a signal from a sensor 1 for measuring atmostpheric pressure and water temperature is input, and a pulse motor 51 for adjusting the opening of the orifice 52. Solenoid valves 12, 13 are provided in a path communicating with a negative chamber 82 of the air valve 8 to adjust the level of the negative pressure. Then, these solenoid valves 12, 13 are opened or closed by a signal from a pressure difference sensor 7 for detecting the pressure difference between the air pressure and the pressure at the benturi 2.