A method for performing processing on a gas sensor having a sensor element containing an oxygen ion conductive solid electrolyte as a major component, said sensor element including: an internal space into which a measurement gas is introduced from an external space; a first electrode formed on a surface of said internal space; a second electrode formed on an external surface of said sensor element; and a reference electrode provided inside said sensor element, said sensor element being configured to be capable of pumping out oxygen in said internal space by applying, across said first electrode and said second electrode, a predetermined voltage determined based on a potential difference between said first electrode and said reference electrode, said sensor element being configured to be capable of stabilizing an oxygen concentration in the vicinity of a surface of said reference electrode by electrically connecting said reference electrode to a positive terminal of a first external DC power supply, electrically connecting said second electrode to a negative terminal of said first external DC power supply, and continuously or intermittently applying a DC voltage across said reference electrode and said second electrode, said method comprising the steps of: a) electrically connecting said reference electrode to a negative terminal of a second external DC power supply, and electrically connecting said second electrode to a positive terminal of said second external DC power supply; and b) in an electrically-connected state created by said step a), applying a DC voltage having a maximum value of 1.4 V to 2.0 V inclusive across said reference electrode and said second electrode for 10 seconds to 1200 seconds inclusive while setting temperature of said sensor element to 700 °C to 850 °C inclusive. an internal space into which a measurement gas is introduced from an external space; a first electrode formed on a surface of said internal space; a second electrode formed on an external surface of said sensor element; and a reference electrode provided inside said sensor element, said sensor element being configured to be capable of pumping out oxygen in said internal space by applying, across said first electrode and said second electrode, a predetermined voltage determined based on a potential difference between said first electrode and said reference electrode, said sensor element being configured to be capable of stabilizing an oxygen concentration in the vicinity of a surface of said reference electrode by electrically connecting said reference electrode to a positive terminal of a first external DC power supply, electrically connecting said second electrode to a negative terminal of said first external DC power supply, and continuously or intermittently applying a DC voltage across said reference electrode and said second electrode, said method comprising the steps of: a) electrically connecting said reference electrode to a negative terminal of a second external DC power supply, and electrically connecting said second electrode to a positive terminal of said second external DC power supply; and b) in an electrically-connected state created by said step a), applying a DC voltage having a maximum value of 1.4 V to 2.0 V inclusive across said reference electrode and said second electrode for 10 seconds to 1200 seconds inclusive while setting temperature of said sensor element to 700 °C to 850 °C inclusive. a) electrically connecting said reference electrode to a negative terminal of a second external DC power supply, and electrically connecting said second electrode to a positive terminal of said second external DC power supply; and b) in an electrically-connected state created by said step a), applying a DC voltage having a maximum value of 1.4 V to 2.0 V inclusive across said reference electrode and said second electrode for 10 seconds to 1200 seconds inclusive while setting temperature of said sensor element to 700 °C to 850 °C inclusive.