PURPOSE:To detect a failure of material easily and quickly, by taking the end of hot steel member as the surface to be inspected after cleaning and smoothing, and etching the said inspected surface with ammonium persulfate. CONSTITUTION:The cast iron piece 1 cut into a given length during casting is carried on the carrying roll 2 ON-line and set to the inspection place provided at a given location in advance. Further, the end 3 of the cast iron piece 1 is adjusted to the surface temperature of 200-400 deg.C. To show the endoplasmic failure on the surface to be tested, dilute ammonium persulfate solution in 5% or lower is poured by using the liquid pouring nozzle 11. This water solution or water pouring amount is good in 10-15cc/cm<2>. Thus, the quick treatment of endoplasm of steel being the tested surface at the range of high temperatures is made by the etching with ammonium persulfate.