Car exhaust shall go through reduction to convert NOx into N2. Since Lem(excessive oxygen) condition and Rich(fuel process) condition are repeated as operating conditions of the engine, NOx purification effect of high efficiency can be hardly obtained just by TWC. Therefore a unit to effectively reduce NOx by coating NOx reserve substance on TWC was developed. The engine adsorbs NO in NOx reserve substance under Lean condition, NO is converted into NO2 in Rich mode, and then reacted with Rh into N2 and CO2.