The resistivity changes of porous sintered SnO2 were measured in an ambient consisting of O-2, NO, NO2, H2O, CO2, and N-2, in which the concentrations of O-2, NO, and NO2 were varied. The adsorption behavior of the adsorbed species related to NO and NO2 was investigated by a temperature programmed desorption, and Fourier transform infrared spectroscopy. The individual contributions of the O-2, NO, and NO2 gas species in the ambient each affected the resistivity change of SnO2 at 550 degrees C. It was considered that Sn-O, Sn-NO, Sn-NO2, and Sn-O2NO sites were formed competitively on SnO2 and that the adsorption and desorption behavior reached equilibrium in a short time. On the basis of the investigation results, a model of the resistivity change was proposed. The resistivity that was calculated using the model, assuming a Langmuir competitive adsorption, corresponds well with the experimental results. Thus, the model is applicable to the estimation of the resistivity change of SnO2 in the atmosphere, in which the concentration of O-2, NO, and NO2 varied.