The reaction of NO with the hydrogen formed by electrolyzing water vapour in a YSZ cell has been applied to detect NO with output signal of magnitude in millivolt or milliampere. The experimental apparatus consisted of two YSZ cells, Pt\YSZ\Pt, which served as an electrolysis cell and an oxygen sensor, respectively. A mixture of 0-3000 ppm NO and 3% H2O in argon was successively fed to the two cells at 800 degrees C. In the upstream cell, the hydrogen formed by electrolyzing water vapour in the sample gas reacted with NO in the sample gas. In the downstream cell : the electromotive force (EMF) value was measured using air as a reference gas. The EMF value of the oxygen sensor was used as a sensor signal, when a current of 6.7 mA was applied to the electrolysis cell. The EMF signal decreased with increasing NO concentration in the sample gas. Furthermore, the current applied to the electrolysis cell was used as another sensor signal, when the EMF value of the oxygen sensor was held at 700 mV. The current signal increased with increasing NO concentration in the sample gas.