PURPOSE: To provide a furnace lid sealing structure, never contaminating an atmosphere in a furnace with atmospheric air, in an atmospheric furnace.

CONSTITUTION: O-rings 5, 1 are arranged in double on an abutting part of a furnace main body 1 to a furnace lid 3 while a gas discharging port 9 is arranged between the O-rings 5, 7. Gas, not contaminating the atmosphere in the furnace, is supplied to the gas discharging port 9 under a condition that the gas is pressurized to a degree of (atmospheric pressure) + (0.1-0.2atm). According to this method, a work is carried into the furnace, the furnace lid 3 is closed to evacuate the inside of the furnace and the supply of Ar gas into the furnace is started while Ar gas is pressurized and supplied between the O-rings 5, 7 through the gas discharging port 9. The inside of the furnace is replaced with the atmosphere of Ar and a space between the O-rings 5, 7 is filled with the Ar gas having the pressure of (an atmospheric pressure + α) whereby the invasion of atmosphere into the furnace can be precluded.