NASICON (Na 3 Zr 2 Si 2 PO 12 )-based potentiometric NO 2 devices, such as Au-attached, (ITO, NaNO 2 )-attached and (ITO, NaNO 2 –Li 2 CO 3 )-attached types, were fabricated and their NO 2 sensing properties were investigated in the range of 2–7ppm NO 2 under 1–50% relative humidity (RH) between 30°C and 150°C. The Au-attached device had a NO 2 sensing capability under wet condition (30% RH) without an auxiliary phase, although it showed no EMF response under dry condition (below 1% RH). This result suggests that NO 2 sensing reaction of the device is closely related with water vapor. To enlarge the NO 2 sensitivity at room temperature, a (metal, nitrite)-attached device was fabricated by combining ITO with NaNO 2 . The EMF change of the (ITO, NaNO 2 )-attached device was estimated to be 43.6mV under 30% RH in the range of 2–7ppm NO 2 and enlarged remarkably in comparison with 17.7mV of the Au-attached device. However, the NO 2 sensitivity of the (ITO, NaNO 2 )-attached device was not only fluctuated by the change of relative humidity, but the EMF values shifted upward with increasing humidity at a fixed NO 2 concentration. It is considered that this originates from the fact that a nitrite auxiliary phase is easy to dissolve in water. Therefore, Li 2 CO 3 with durability to water vapor was melted together with NaNO 2 and used as the auxiliary phase of the (ITO, NaNO 2 )-attached device. As the result, it was found that the EMF change of the (ITO, NaNO 2 –Li 2 CO 3 )-attached device was constant almost without depending on humidity. The EMF values of the device were also proportional to the logarithm of NO 2 concentration and well consistent with theoretical values obtained from the Nernstian equation.