PURPOSE: To stabilize idle operation by setting a variable venturi carburetor to the rich side at the time of idle, when an automatic transmission is in a nondrive position or nonbrake operation, and supplying secondary air when it is in a drive position and brake operation.

CONSTITUTION: A control circuit 50 judges to be at the time of idle when a throttle valve 11 is fully opened to turn a throttle switch 42 ON, and the actual number of revolutions from a revolution sensor 43 is less than a prescribed value, and stops the air-fuel ratio feedback control of a variable venturi carburetor 10 based on the detected value of an O2 sensor 41. In other words, an air bleed control valve 22 which controls the air bleed to a measuring jet 15 is fully closed to set the air-fuel ratio to the rich side. And when an automatic transmission 44 is in a drive range and a brake switch 45 is turned ON, an air introducing valve 36 is opened to prevent the overrich due to the decrease in the number of revolutions, so that secondary air is introduced.