In this invention, an EMF oxygen sensor is subjected to an activation process applying unidirectional voltage between an atmosphere electrode and an exhaust electrode thereof. A control device controlling the oxygen sensor in which a voltage was applied with the atmosphere electrode being positive, additionally applies unidirectional voltage between the electrodes to make the atmosphere electrode positive, for example, when the oxygen sensor was used under an environment in which the air-fuel ratio of the internal combustion engine was rich relative to the theoretical air-fuel ratio. Conversely, a control device controlling the oxygen sensor in which a voltage was applied to make the atmosphere electrode negative, additionally applies unidirectional voltage between the electrodes to make the atmosphere electrode negative, for example, when the oxygen sensor was used under an environment in which the air-fuel ratio was lean relative to the theoretical air-fuel ratio.