Activation treatment for a zirconia oxygen sensor is performed by applying alternately a treating voltage, which can make an oxygen partial pressure at the electrode-zirconia interface of a sensor equal to or higher than a critical oxygen partial pressure value (10.sup.-34 atm) not to cause blackening, between a pair of electrodes of the sensor at temperatures of 500.degree. to 800.degree. C. As a result, the treatment current flowing through the sensor can always be held below the critical level, whereby the sensor can be activated without causing blackening and other physical structural changes to occur in the zirconia sensing element. If the sensor is to be used in an A/F ratio control or the like on an internal combustion engine, it can be rendered active at all times by the activation treatment during or before or after the control.