An air-fuel ratio control device capable of preventing torque shocks liable to be produced in an engine when an air-fuel ratio is changed in order to carry out switching between rich combustion and lean combustion wherein an electronic control unit (ECU) of the device carries out, while the engine (E) is in an operating state other than a particular operating state, on-off duty control of a valve (30) disposed in a first by-pass passage (31) for by-passing a throttle valve (7) in accordance with the difference between a target air-intake volume needed to control the engine to a lean air-fuel ratio that is calculated in accordance with the opening of the throttle valve detected by means of a throttle sensor (14) and an air-intake volume detected by means of an air-flow sensor (11), while biassing a by-pass valve (42) for opening a negative pressure responsive valve (41) disposed in a second by-pass passage (43), whereby a predetermined volume of air can be supplied to the engine via the throttle valve, and the first and second by-pass passages. While the engine is in a particular operating state, an air supply through the throttle valve is permitted which is sufficient to control the engine to a rich air-fuel ratio status with an air-supply through the by-pass passages being prevented.