In an apparatus for controlling an air/fuel ratio of a general-purpose internal combustion engine using mixed fuel containing alcohol and gasoline and operated at a desired engine speed inputted by the operator while a throttle opening is regulated such that a detected engine speed converges to the inputted desired engine speed, a fuel injection amount for mixed fuel determined based on fuel injection amount characteristics is increased/decreased when a load is kept constant and the output air/fuel ratio (at which an output of the engine becomes maximum) is estimated. And the fuel injection amount is corrected based on the estimated air/fuel ratio, thereby enabling to estimate the output air/fuel ratio without using an expensive air/fuel ratio sensor and to feedback-control the fuel injection amount such that the engine is operated at the output air/fuel ratio.