A gas sensor is provided which constitutes a pump cell 3 and a sensor cell 5 using a single solid electrolyte body 2, a pump electrode 30, a sensor electrode 50, and a reference electrode 80 and is designed to decrease power consumed by a heater and permit a size thereof to be reduced. A ratio of a minimum distance L2 between the pump electrode 30 and the sensor electrode 50 to a thickness d of the solid electrolyte body 2 is set to be three or more, thereby enabling the gas sensor 1 to make the pump cell 3 and the sensor cell 5 using the single solid electrolyte body 2, the pump electrode 30, the sensor electrode 50, and the reference electrode 80. Only either of the gas chamber or the reference gas chamber is, therefore, located between the solid electrolyte body and the heater, thereby decreasing distances of the pump cell and the sensor cell to the heater. This facilitates the ease with which the heater heats up the pump cell and the sensor cell.