Abstract: A gas sensor includes a measurement chamber, a reference gas chamber, a first cell, and a second cell. The first cell includes a first electrode in the measurement chamber and a second electrode in the reference gas chamber. The second cell includes a third electrode in the measurement chamber and a fourth electrode in the reference gas chamber. A gas sensor detects a first current that flows between the first electrode and the second electrode, detects a second current that flows between the third electrode and the fourth electrode, calculates a concentration of a specific gas component based on a detected value of the second current, estimates a change amount of oxygen concentration in the reference gas chamber based on a detected value of the first current, and corrects the detected value of the second current based on an estimated value of the change amount of the oxygen concentration.