Abstract: Herein disclosed is a method and an apparatus for judging the fitness of a steel composition for a particular casting practice in order to prevent excessive segregation of manganese and phosphorous in the course of the casting. We have discovered that the simultaneous segregation of manganese and phosphorous tends to cause the formation of bainite and/or martensite in steel plate product during the cooling of the steel plate after rolling. When the manganese and phosphorous concentrations satisfy the following requirement (1):Mn (%) .times. P (%) <0.60/f . . . . (1)wherein f denotes the segregation factor of the particular casting practice, regarding the product of Mn (%) and P (%), the formation of bainite and/or martensite is prevented. Accordingly, it is possible to predict the fitness of the molten steel to the particular casting practice.