Recently ammonia has attracted attentions as a candidate of energy (hydrogen) carrier, because it has a high energy density and a high hydrogen content, and carbon dioxide is not released when hydrogen is produced by ammonia decomposition. However, the current major process of ammonia production, the Haber-Bosch process, is operated under high temperature and high pressure. Therefore, Haber-Bosch process consumes a great deal of energy. Based on such background, to reduce energy consumption, a process and a substance that can catalyze ammonia synthesis under mild conditions (low temperature and low pressure) are strongly demanded. Here we show praseodymium oxide supported ruthenium catalyst (Ru/Pr₂O₃) as a candidate of new generation ammonia synthesis catalyst which shows superior ammonia synthesis activity compared with that of the supported Ru catalyst already reported. In order to investigate the origin of high catalytic activity of Ru/Pr₂O₃, we employed several characterization techniques. We found that Ru is deposited over the surface of the Ru/Pr₂O₃ in low-crystallite nano-layers and the catalyst is strongly basic. We considered that such unique surface structural and electronic characteristics synergistically accelerate the rate-determining step of NH₃ synthesis, cleavage of the N≡N bond.