A process for producing an oxidizing catalyst includes the steps of thermally treating a double-chained-structure type clay mineral including as a constituent Mg and/or Al in a temperature range of from 400 to 800 °C to modify the double-chained-structure type clay mineral to an amorphous clay mineral at least partially having an amorphous structure, substituting a transition metal element for at least a part of the constituent Mg and/or Al of the amorphous structure in a ratio of from 0.05 to 0.80 with respect to a total number of atoms of the constituent Mg and/or Al, and loading at least one of platinum-group elements on the substituted amorphous clay mineral. Note that, when the substituted amorphous clay mineral is subjected to an acidifying step followed by a salts removing step, it is unnecessary to precisely substitute the transition metal element for the constituent Mg and/or Al in the ratio. The resulting oxidizing catalyst can oxidize and decompose HC and CO included in exhaust gases even at low temperatures, and can inhibit the sulfates production even at high temperatures so as to reduce the emission of particulate floating materials. These advantageous effects maintain even after a durability test.