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To derive an effective technique for reducing a greenhouse gas nitrous oxide (N2O) emitted from automobiles, we prepared experimental three-way catalysts carrying various types and quantities of precious metals, and investigated their N2O generation and conversion characteristics. In view of previous reports on increased N2O emissions from in-use automobiles, we deactivated the catalysts in a rapid aging test, and observed the effect of catalyst deactivation on N2O generation and conversion by the catalysts. We found that the concentrations of generated N2O decreased as the quantities of precious metals carried by catalysts were decreased and that, accordingly, these low-carrying catalysts are more advantageous from the standpoint of reducing N2O generation. However, the concentrations of generated N2O increased as the catalysts were deactivated.