There is provided an air-fuel ratio control device comprising: fuel supply means; engine operation state detection means; air-fuel ratio information detection means; and an electronic control unit, the electronic control unit being configured to determine a fuel injection amount which realizes a target air-fuel ratio while using engine load information by the engine operation state detection means on the basis of air-fuel ratio information by the air-fuel ratio information detection means, and then output a driving signal to the fuel supply means, to execute feedback control mainly at an excessively rich side air-fuel ratio, characterized in that the air-fuel ratio information detection means is an exhaust gas temperature sensor, and while estimating an air-fuel ratio at this time on the basis of detected exhaust gas temperature information by a predetermined obtaining method, the air-fuel ratio is used for the control.