To prevent bumping of molten steel generated with oxygen supplied from an injection pipe from being carried out by a method wherein a jet immersion depth is calculated in response to a detected position of molten metal level and an injection speed, the injection speed is adjusted to control a jet accepting depth and further a distance ranging from a bottom part of a furnace to a bottom part in a recess portion.

O2 is blown against molten steel in a furnace body from a lance 8 in a form of jet to form a recess portion 13 indented only by a jet immersed depth L from a molten metal surface 12 of molten steel. Then, a jet injection speed V attained from a lance 8 is detected by an injection speed sensor 26, a position of molten metal surface of molten steel is detected by a molten steel surface position sensor 20, a jet immersion depth L is calculated by a calculator 30, a control device 28 controls a flow rate adjusting valve 24 in response to the jet immersion depth L attained by the calculator 30 and then controls the jet immersion depth L. In this case, the jet immersion depth L is set to become such a value as one in which a distance L1 ranging from the bottom part of a furnace to the bottom part of the recess portion 13 is more than 0 and less than 300 mm.