To make it possible to not only close but also open a discharge opening in a short time and with ease, and moreover, in safety and definitely.

A coating layer is formed on the inner peripheral surface of a discharge opening 12 formed on a furnace bottom 11 while a porous brick- made cylinder 13, which is filled up with a sand-like filler 14 inside, is inserted thereinto. A pipeline 16, which supplies oxygen gas during opening, is provided inside the cylinder 13. After having discharged a molten article, the cylinder 13, which is filled up with the sand-like filler 14 inside, is inserted into the emission opening 12, thereby closing the opening. Before opening, oxygen gas is supplied to the cylinder 13, which is fitted into the discharge opening 12, thereby melting a sintered layer of the filled sand produced at a contact site with a high temperature dissolved article or cooled skull, slag and so forth. Then, an open close gate 17 is opened, thereby allowing the sand-like filler 14 filled up in the cylinder 13 and the dissolved article to drop by self-weight so that it may be opened.