PURPOSE: To improve decarbonizing oxygen efficiency, to reduce oxygen concn. in slag and molten steel and to achive improvement of Fe and Mn yield by making oxygen flow rate the value corresponding to decarbonizing velocity while adjusting top-blowing oxygen original pressure and atmospheric pressure in a converter.

CONSTITUTION: A Laval nozzle is set in a top blowing lance, and while adjusting the oxygen original pressure P0 and the atmospheric pressure P2 in the converter so that Mach number M2 shown in the equation II for oxygen jet injected from the above nozzle satisfies the inequality I, the refining is executed. Wherein, in the equation and the inequality, D1: throat diameter of the Laval nozzle, D2: outer diameter of the above nozzle, k: ratio of sp. heat of oxygen (=1.4), P0, P2: absolute pressures (kg/cm2 abs). That is, in the case of intending to reduce the oxygen flow rate at the time of end stage in the refining, the above pressure P2 in the furnace is lowered, and on the other hand, at the time of initial stage and intermediate stage, the above pressure P2 is raised, and while raising the oxygen flow rate, the refining is executed.