A gas sensor is equipped with a sensor device made of a stack of a solid electrolyte body, a sensor electrode, a reference electrode, a first insulator, a second insulator, a gas chamber, a reference gas duct, a heater, and a heat transfer member. The heater has a heating element at least partially overlapping the sensor electrode and the reference electrode. The heat transfer member is made of a dense metallic oxide material which blocks passage of measurement gas therethrough. The heat transfer member is held between the sensor electrode and the first insulator in which the heater is embedded within the gas chamber and works to facilitate transfer of thermal energy, as generated by the heater, to the solid electrolyte body, the sensor electrode, and the reference electrode. This results in enhanced thermal conductivity of the sensor device and achieves quick activation of the sensor device.