PURPOSE: To perform the switching of the air-fuel ratio rapidly and to improve the acceleration responsiveness by forming an air introduction passage bypassing a venturi section and a throttle valve and by providing a closing valve opened only during an idle operation on said passage.

CONSTITUTION: This control device is constituted by providing an air introduction passage (A) bypassing a suction air passage from a venturi section 12 to a throttle valve 16 of a carburetor proper 10. This air introduction passage (A) is composed of an air take out pipe 18 and air feed pipe 20, and a solenoid closing valve 21 is fitted between both pipes 18, 20 so that it can be opened or closed in accordance with signals from a control circuit 22. During the idle operation of the engine, the closing valve 21 is opened by the control circuit 22, and the air introduction passage (A) is opened, thereby increasing the air quantity, i.e., the air fuel ratio is controlled at a lean side. When the throttle valve 16 is rotated, the closing valve 21 is closed, and the air-fuel ratio is controlled at a rich side.