An air-fuel ratio control apparatus capable of preventing production of a torque shock which is liable to be caused in an engine when the air-fuel ratio is varied for a change between rich combustion and lean combustion. When the engine is in any other operating state than a specific operating state, an electronic control unit of the apparatus subjects a valve, which is disposed in a first bypass line for bypassing a throttle valve to on/off-duty control in accordance with the difference between a target suction air amount necessary for control for a lean air-fuel ration, computed according to a throttle valve opening detected by a throttle sensor, and a suction air amount detected by an airflow sensor, and urges a bypass valve for opening a negative-pressure responsive valve in a second bypass line. As a result, a predetermined amount of air is allowed to be supplied to the engine through the throttle valve and the first and second bypass lines. In the specific operating state, enough air supply through the throttle valve for control for a rich air-fuel ratio is allowed while preventing air supply through the bypass lines.