Solid-state electrochemical devices were fabricated by combining an Na+ conductor (NASICON) with auxiliary phases of NaNO2-M2CO3 (M = Na or Li). These devices showed good sensing characteristics to NO2 in synthetic air. The use of NaNO2-Li2CO3 (9:1 in molar ratio) gave quite excellent sensing properties especially to extremely dilute NO2. The electromotive force of the device followed the Nernst equation in the wide NO2 concentration range of 0.005 ppm (5 ppb)-200 ppm at 150-degrees-C, its slope suggesting the one-electron reduction of NO2. The 90% response times of the sensor to 10 ppm and 5 ppb were ca. 8 s and ca. 3 min, respectively. In addition, the device was found to be utterly insensitive to CO2 up to 40 vol%.