PURPOSE: To facilitate air fuel ratio control by setting the ratio between the volume of air going through a fuel supply unit and a secondary volume of air by an output signal of an actuator.

CONSTITUTION: A secondary air passage 13 is provided with a bypass throttle valve 14. A diaphragm 18 of a negative pressure control valve 4 is provided with a secondary air control lever 24 through a control valve link 21. An actuator control chamber 27 for a pressure regulator valve 8 is communicated to an actuator 6 through a control pressure passage 9. A regulator valve 31 is firmly mounted to a regulator valve diaphragm 26. By the action of the regulator valve 31, a requlator valve seat 32 for a pressure requlating chamber 33 provided in an upper part, is opened and closed. An actuator 6 consists of a three-way solenoid valve and communicates an output pipe 35 to a negative pipe 36 or to an ambient pipe 37 by a signal of ECU5.