A gas sensor includes a solid electrolyte, a gas chamber, a reference gas chamber, a pump cell, a monitor cell, and a sensor cell. The gas chamber has a spatial width W0 constant in a width direction W orthogonal to the direction of flow of a gas in a position where the pump electrode, the monitor electrode, and the sensor electrode are provided on the solid electrolyte. An amount of shift ΔX1 of a central position O2 of a gap S in the width direction W between the monitor electrode and the sensor electrode from a central position O1 in the width direction of the pump electrode has relationship of ΔX1≤¼ W1 where the pump electrode has a width W1. In addition, positions ΔY1 of a of a side surface of the monitor electrode and of a side surface of the sensor electrode from the central position O1 in the width direction W of the pump electrode have relationship of ΔY1≤½ W1.