PURPOSE: To surely and simply know the changing time of the porous plug by arranging the material having the melting point lower than the melting point of the molten metal to be treated to the lower side of the opposite working surface side.

CONSTITUTION: The porous fire proofing material 1 is arranged in the metal case 2 of the porous plug 10. If the porous fire proofing material 1 is damaged during operating the porous plug 10 in the actual furnace, the temperature of the opposite working side is increased, the low melting point material 5 is molten. The molten low melting point material 5 flows to the gas pool and seals the gas introducing tube 3 at the seal adjusting material 6 of the outlet side of the gas introducing tube 3. Therefore, the operator can know the time for changing the porous plug surely and simply.