PURPOSE: To enable discharged gas generated in an enclosure to be economically and effectively sealed and then to improve a working environment or an efficiency of processing of discharged gas by a method wherein a top opening port of the enclosure is provided with small doors which can be opened or closed.

CONSTITUTION: An enclosure 11 covering a side part and an upper part of an electric furnace is built. The enclosure 11 is provided with a front opening 21 of which part is extended up to the top surface and a top surface opening 22. The front surface opening 21 is provided with large doors 31 and 32 which can be opened or closed. The top surface opening 22 is provided with an air curtain inside the top surface of the enclosure 11. The top surface opening 22 is provided with small doors 41 and 42 which can be opened or closed. To a side surface of the enclosure 11 is connected a discharging duct 51. The discharging duct 51 is connected to a dust collector so as to suck the discharged gas generated in the enclosure 11 into the dust collector through the discharging duct 51.