To prevent nonferrous metals such as lead and the like contained in a molten steel from intruding through a gas blowing hole as much as possible and facilitate to exclude the non-ferrous metals by raising gas pressure even if the nonferrous metals have entered by forming the gas blowing hole in a funnel like shape or an umbrella like shape.

A gas blowing hole 10 is formed in a funnel like or umbrella like shape. The gas blowing hole 10 having such a funnel like or umbrella like shape can blow gas at a higher speed at an upper side of an outer peripheral portion of the blowing hole than at a lower side so that molten lead having a tendency to adhere onto a top side of an outer peripheral portion of the blowing hole is scattered into outside stumping material and becomes easy to be absorbed. On the other hand, droplet molten lead adhering on a lower side of the outer peripheral portion of the blowing hole is difficult to be scattered but easy to move into the stumping material in vicinity due to vibration of a whole furnace body or vibration during gas blowing operation, and therefore the molten lead generally becomes difficult to adhere onto the peripheral portion of the hole. By raising blow gas pressure the lead intruding into the hole can be easily excluded.