A gas sensor having a sensor element that includes an inner space for introducing a measurement gas therein from an external space and a pump cell which has a first electrode formed on a surface of the inner space and a second electrode formed in a space different from the inner space and is provided to pump oxygen out of the inner space by applying a predetermined voltage between the first electrode and the second electrode. Assuming that the length of the inner space in a short-side direction of the sensor element as viewed from the front end portion side thereof is x1 and the length of the inner space in a longitudinal direction of the sensor element as viewed from the front end portion side thereof is x2, the following inequality is satisfied: 0.05≦x1/x2≦0.25.