A system for controlling an air/fuel ratio for an internal combustion engine, using an oxygen concentration sensor having a first chamber for introducing therein exhaust gas of the engine and a second chamber for introducing therein reference ambient air to detect oxygen content in the exhaust gas. In order to eliminate exhaust gas pulsation which could affect on detection accuracy, it is firstly determined if engine operation is in a transient state or in a normal state. Then the detected oxygen concentration value in the preceding cycle is added to the product obtained by multiplying the deviation between the values in the preceding and current cycles by a coefficient .alpha.. The coefficient is made different in the transient state and in the normal state. In the normal engine operation state, the coefficient .alpha. is determined from engine speed and load.