To provide a reliable technique for stabilizing a molded surface coating film in the casting of ceramic or the like or die cast alloy manufacture.

The molded casing 23 by forging of the ceramic or the like, die cast alloy forging, thixomold.forging, press forging machining or the like is cleaned by a supercritical cleaning medium circulation system having a high performance filter 10 interposed in the mid-way of the circulation and constituted so as to form the circulation flow into the continuous flow to degrease infiltrated oil portion and a stable oxide coating film is formed on the surface of the molded casing 23 by allowing the metal portion of the molded casing 23 to be cleaned to oxidation react with a super critical gas under the super critical environment.