was 10 oxygen was present at a level of 100 ppb and moisture was present at a level of 100 ppb the other conditions were identical to those in embodiment 2 the results of an xps analysis of the passive film formed under the above conditions are shown in fig4 as is clear from fig4 the passive film formed under the above conditions has chromium oxide as a chief component thereof that is to say the value of cr fe is 5 or greater and the thickness of the passive film was 5 nm or more using sus316l stainless steel electrolytic polishing was conducted in a manner identical to that of embodiment 1 this was designated sample 1 next baking was conducted in a manner identical to that of embodiment 1 heat treatment was conducted at a temperature of 500 u00b0 c and for a period of 1 hour in an atmosphere of a gas in which hydrogen and oxygen were added to an argon gas base so that the hydrogen concentration was 10 and oxygen was present at a level of 100 ppb and a passive oxide film was thus formed this was designated sample 2 sus316l stainless steel was subject to composite electrolytic polishing electrolytic polishing was conducted so as to remove the layer altered by working on the surface and baking and heat treatment were conducted in a manner identical to that of sample 2 and a passive oxide film was formed this was designated sample 3 the results of an xps analysis of the surface layers of samples 1 2 and 3 are shown in fig7 a b and c respectively as shown in fig7 oxide films having.