This research is aimed at development of the catalyst for gasoline automobiles which uses only palladium (Pd) among platinum group metals (PGMs). And the conformity emission category aimed at LEV III-SULEV30. For evaluation, the improvement effect was verified for 2013 model year (MY) ACCORD (LEV II-SULEV) as the reference. As compared with Pd-rhodium (Rh) catalyst, a Pd-only catalyst had the low purification performance of nitrogen oxides (NOx), and there was a problem in the drop in dispersion of Pd by sintering, and phosphorus (P) poisoning. The following was performed in order to improve these. 1) NOx purification performance was improved using an enhanced basicity support material by promoting the dissociation of N and O in NOx 2) sintering of Pd was controlled using the anchoring effect of praseodymium-zirconia (PrZrOx) 3) poisoning inhibition of Pd and the support material was carried out utilizing the P trapping effect of praseodymium (Pr) Tail pipe emission was set to the LEV III-SULEV30 regulation conformity level by these improvement and optimization.