An air/fuel ratio monitoring system in an IC engine, using an oxygen sensor of the concentration cell type which has an inner electrode layer, a microscopically porous layer of oxygen ion conductive solid electrolyte, and an outer electrode layer to be exposed to the exhaust gas and exhibits a sharp change in the level of output voltage in response to a change in the air/fuel ratio in the engine across the stoichiometric ratio. To ensure accurate monitoring of the air/fuel ratio even though an average level of the sensor output changes for various reasons such as aging of the sensor, the monitoring system produces a variable reference voltage. The output of the oxygen sensor is compared with this reference voltage by first adding or subtracting a fixed voltage to the output voltage of the sensor, depending on the result of comparison between the sensor output. The resultant reference voltage and voltage is then smoothed in an RC circuit. To prevent misjudgement of the air/fuel ratio by unintentional intersection of the sensor output voltage attenuating after responding to a change in the air/fuel ratio across the stoichiometric ratio and the reference voltage, the system includes a control means for varying the time constant at the voltage-smoothing operation according to the manner of a change in the sensor output voltage.