A gas sensor including a reduction section (18) for reducing NO2 contained in exhaust gas to NO. The reduction section (18) is provided on the upstream side of a first diffusion resistor section (103) which limits the flow of the exhaust gas into a first measurement chamber (101). When NOX passes through the first diffusion resistor section (103), NO2 which has a greater molecular weight than NO has a lower degree of diffusion. Since NO2 is reduced to NO at the reduction section (18), the exhaust gas passing through the first diffusion resistor section (103) hardly contains NO2. Therefore, the speed of flow of NOX through the first diffusion resistor section (103) is not limited by NO2, whereby sensitivity for detection of NOX can be improved.