PURPOSE: To make it possible to improve exhaust gas performance and maintain stabilization for a long time by providing an alumina coat on a steel plate and supporting precious metal.

CONSTITUTION: An attempt is made to heat a carbon steel plate 1 which contains at least 10% of chrome and at least 2% of alumina in order to produce a fine alumina coat 12 on the steel surface and support precious metals on the surface, such as platinum group elements. This construction makes it possible to oxidize carbon monoxide produced by imperfect combustion when petroleum fuels are burnt, and various kinds of aldehyde and unburnt hydrocarbon, thus exerting oxidizing and cleaning performance into carbon dioxide. In addition, an alumina coat produced on the surface is provided with an extremely excellent high temperature and oxidation proof quality. It is, therefore, possible to exert stabilized performances for a long time.