A gas sensor includes: a sensor element including an element body which is internally provided with a measurement-object gas flow portion that introduces a measurement-object gas and causes the measurement-object gas to flow, a measurement pump cell having an inner measurement electrode disposed in a measurement chamber and an adjustment pump cell that adjusts an oxygen concentration in an oxygen concentration adjustment chamber, a necessity determiner that performs a refresh necessity determination process of determining necessity of a refresh of the sensor element based on at least one of undershoot or overshoot at a time of rapid change in a measurement pump current; and a refresh controller that, upon a determination that the refresh is necessary in the refresh necessity determination process, performs a refresh process including at least one of an adjustment pump control process during refresh or a measurement pump control process during refresh.