An air-fuel ratio controlling apparatus for internal combustion engines including an oxygen sensor of hermetic type is disclosed. A pump cell of the oxygen sensor is connected with a power supply through a switch circuit for switching the direction of current flow. A control unit computes a setting of oxygen concentration of a reference gas layer of an exhaust gas sensor in accordance with the operating parameters. By a setting interruption, the control unit switches the switch circuit toward exhausting direction, drives the power supply and applies the exhaust current to the pump cell. The completion of exhaustion is detected, the switch circuit is switched to the suction, and the power supply is driven to apply the suction current to the pump cell. The setting is compared with the integration value of the suction current by a comparator circuit, or the suction current is fixed to control the drive time of the power supply. Alternatively, the current corresponding to the setting is applied for a predetermined time thereby to set the oxygen concentration of the reference gas layer to a setting in accordance with the operating parameters of the engine.