An air-fuel ratio control system for an internal combustion engine including a carburetor having a main fuel system, a slow fuel system, a main bleed air passage leading to the main fuel system a main bleed air control passage leading to the main fuel system, a slow bleed air passage leading to the slow fuel system, and a slow bleed air control passage leading to the slow fuel system, and a valve for controlling the amount of bleed air supplied through the main and slow bleed air control passages. An electronic control unit controls the operation of the valve and is capable of carrying out a feed-back control for controlling the operation of the valve on the basis of a signal from the oxygen sensor so as to bring the air-fuel ratio into a theoretical value, and which determines a control value in such a manner that a feed-back control value is learned and stored to provide a learned value which is substantially constant with respect to fuel of a constant calorific value, and the stored learned value is reflected to a next control value. Thus, it is possible to perform a prompt and stable control of air-fuel ratio in accordance with a variation in calorific value of fuel and to stably and finely vary the amount of bleed air.