PURPOSE: To lower the flowability of slag and to inhibit the outflow of the slag into a ladle by adding a material contg. carbon such as coke or graphite as a reducing agent to the slag so as to reduce iron oxide in the slag and to lower the total iron content in the slag.

CONSTITUTION: After blowing in a converter is stopped, a material contg. carbon such as coke or graphite as a reducing agent is added to slag produced by refining in the converter. Iron oxide in the slag is reduced and the total iron content in the slag is nearly halved, so that flowability of the slag is lowered and the outflow of the slag into a ladle is inhibited.