With the increasing number of automobiles, the worldwide problem of air pollution is becoming more serious. The necessity of reducing tail-pipe emissions is as high as ever, and in countries all over the world the regulations are becoming stricter.The emissions at times such as after engine cold start, when the three-way catalyst (TWC) has not warmed up, accounts for the majority of the emissions of these pollutants from vehicles. This is caused by the characteristic of the TWC that if a specific temperature is not exceeded, TWC cannot purify the emissions. In other words, if the catalyst could be warmed up at an early stage after engine start, this would provide a major contribution to reducing the emissions. Therefore, this research is focused on the substrate weight and investigated carrying out major weight reduction by making the porosity of the substrate larger than that of conventional products. A lightweight substrate achieving about 31% weight reduction compared to conventional technology, and a conventional substrate with the same cell structure were both mounted into vehicles and a comparison performed. The results showed that, for the lightweight substrate, the time taken for light-off at which it purified the emissions was reduced compared to conventional products.