Catalysts contain noble metal-containing particles that has a composition except oxygen represented by a formula Pt.sub.XRu.sub.YA.sub.ZSn.sub.S and exhibits a mean particle diameter of from 0.5 nm to 10 nm, both inclusively. A is at least one element selected from a group consisting of Rh, Au, Pd, Ir and Os, and X, Y, Z and S are atomic ratios satisfying relations 30.ltoreq.X .ltoreq.70, 30.ltoreq.Y.ltoreq.70, 0.ltoreq.Z.ltoreq.40, 0.5.ltoreq.S.ltoreq.8 and X+Y+Z+S=100.