PURPOSE:To easily and speedily decide the factor of the surface flaw of a steel material without any error by contrasting a normal and an abnormal part by light emission spectrum intensity as to a specific element which is assumed previously to be the estimated cause of a flaw part. CONSTITUTION:The detection area of a light emission spectral analyzing device is scanned continuously crossing a linear flaw 2 present on the surface of the steel material 1. The normal part and abnormal part are contracted with each other by the light emission spectrum intensity as to the specific element which is assumed to be the estimated cause of the flaw part. When the measured intensity of the flaw part is nearly equal to the measured intensity of the normal part, the flaw is considered to be a mechanical flaw generated in a rolling process, etc., and when the both are not equal, it is decided that the flaw is caused by surrounding refractories, powder, or inclusion in a steel ingot manufacturing process. Consequently, the cause of the surface flaw is decided easily and speedily over a wide visual field without any error.
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