Manganese oxides show high catalytic activity for CO and HC oxidation without including platinum group metals (PGM). However, there are issues with both thermal stability and resistance to sulfur poisoning. We have studied perovskite-type YMnO₃ (YMO) with the aim of simultaneously achieving both activity and durability. This paper describes the oxidation activity of PGM-free Ag/i-YMO, which is silver supported on improved-YMO (i-YMO). The Ag/i-YMO was obtained by the following two methods. First, Mn⁴+ ratio and specific surface area of YMO were increased by optimizing composition and preparation method. Second, the optimum amount of silver was supported on i-YMO. In model gas tests and engine bench tests, the Ag/i-YMO catalyst showed the same level of activity as that of the conventional Pt/γ-Al₂O₃ (Pt = 3.0 g/L). In addition, there was no degradation with respect to either heat treatment (700°C, 90 h, air) or sulfur treatment (600°C to 200°C, total 60 h, 30 ppm SO₂). X-ray photoelectron spectroscopy (XPS), X-ray absorption fine-structure spectroscopy (XAFS) and transmission electron microscope (TEM) analysis showed that the silver in Ag/i-YMO was not substituted into the i-YMO, and existed as metallic silver with a particle size of 3 nm to 80 nm on the i-YMO surface. First-principles calculation was performed for the bulk model to investigate the catalytic activity of the silver at the interface between these silver particles and the i-YMO. The results showed that the d-band center of this silver is close to that of PGM. The oxidation activity of Ag/i-YMO was mainly enhanced by this highly active silver.