With the increasing number of automobiles, the problem of air pollution from the exhaust gases of automobiles has become a critical issue. The principal gases that cause air pollution are nitrogen oxide or NO$_x$(NO and NO$_2$), and CO. Because NO$_x$ gases cause acid rain and global warming and produce ozone(O$_3$) that leads to serious metropolitan smog from photochemical reaction, they must be detected and reduced. Mixtures of WO$_3$ and $In_2O_3$(WO$_3$:$In_2O_3$=10:0, 7:3, 5:5, 3:7, and 0:10 in wt.%), which are NO$_x$ gas-sensing materials, were prepared, and thick-film gas sensors that included a heater and a temperature sensor were fabricated. Their sensitivity to NO$_x$ was measured at 250$sim$400$^{circ}C$ for NO$_x$ concentrations of 1$sim$5 ppm. The $In_2O_3$ thick-film sensor showed excellent sensitivity($R_{gas}/R_{air}$=10.22) at 300$^{circ}C$ to 5-ppm NO. The response time for 70 % saturated sensitivity was about 3 seconds, and the sensors exhibited very fast reactivity to NO$_x$.