Use of a metallic material containing chromium as a substrate of an exhaust gas purifying catalyst has a problem that the chromium contained in the substrate migrates to a catalytically active component and reacts with the catalytically active component to reduce exhaust gas purification performance. Thus, a film which inhibits the chromium contained in the substrate from migrating is arranged on the substrate's surface. The film is desirably formed by oxidizing a substrate in the air. It is also desirable that a substrate containing aluminum therein be oxidized to cause aluminum contained in the substrate to separate out and thereby form an alpha-alumina film. The film is preferably such that, when the exhaust gas purifying catalyst is heated at 850° C. in the air for 300 hours, the amount of chromium migrated to the catalytically active component is controlled to 0.5 percent by weight or less based on the catalytically active component.