PURPOSE: To make a stable blackening treatment possible, by treating the metal tube containing a specific quantity of Cr at a high temperature under the atmosphere of specific condition and forming oxidized film essentially composed of a specific thickness of Cr on the surface of the metal tube.

CONSTITUTION: Metal tube composed of alloy containing ≥5wt% Cr, is heated at a high temperature under wet hydrogen atmosphere of 10-12W10-18atm oxygen partial pressure and dense oxidized film of 0.2W3μ thickness essentially composed of Cr, is formed on the surface of the metal tube. Producing quantity of Cr2O3 at blackening treat time, is sufficiently produced by containing ≥5% of Cr. Adequate oxidation speed is obtained by limiting the oxygen partial pressure in a fixed range. The deflective oxidation unenness at the blackening time is sharply decreased by using the metal obtained in this way and the blackened film having each resistance of corrosion and oxidation is obtained.