A new instrument for simply and rapidly determining an ammonia concentration in a gaseous mixture has been developed. The principle is based on the fact that ammonia reacts with NO exactly at 1:1 mole ratio in the presence of an appropriate catalyst: NH3+NO+ 1/4 O2=N2+ (3)/(2) H2O. A sample gas containing NH3is mixed with NO and made to contact with a catalyst capable of promoting reaction of NH3and NO to form N2and H2O in an oxidative atmosphere at an elevated temperature, preferably, 300°–400 °C. The catalyst capable of conducting the reaction is composed of TiO2and at least one of the oxides of V, Fe, Cu, W, Mo. The concentration of NH3is determined by measuring concentrations of NO before and after the reaction and calculating the consumption of NO. For the analysis of NO in the gas, a chemiluminescence‐type analyzer can be preferably used. According to the present method, ammonia in a sample gas can be analyzed within a few minutes without any influence of various coexisting gas components.