Exhaust gas from a diesel engine is one of the main causes of air pollution. It is difficult to reduce NO under the excess O-2 concentration so that effective method of NO reduction has not been developed. In this study, nickel on the porous glass (VYCOR(R) glass) was examined as a catalyst for NO reduction. This catalyst was effective for NO reduction where 35% of NO was converted to N-2 under excess O-2 concentration (3%) in the tubular fixed-bed reactor. Adsorption rates of NO and O-2 gases were measured in order to examine the mechanism of selective reduction of NO by porous VYCOR catalyst. It was proved that NO adsorption rate was much higher than O-2 adsorption rate, so that difference of adsorption rate was one of the main causes of NO reduction. As a short lifetime of the-catalyst was the serious disadvantage of the tubular fixed-bed reactor, another type of catalytic reactor was proposed and tested. It is a membrane reactor consisting of a porous VYCOR catalyst tube and a quartz tube. NO reduction under excess O-2 concentration was also achieved in the membrane reactor. Maximum conversion from NO to N-2 was 45%. The possibility of longer lifetime catalyst could be found.