PURPOSE: To protect an atmosphere protective valve from the dispersion of molten metal and the heat of molten metal and eliminate the occurrence of trouble during operation by a method wherein a through device is lifted by operating a lifter prior to the discharging of the molten metal and a heat resistant sleeve is inserted into the opening of the atmosphere protective valve.

CONSTITUTION: Prior to the discharging of molten metal from the discharging port 12 of the trough 7 of a melting furnace, the valve gate 14 of an atmosphere protective valve 13 is opened and a lifter 16 is operated to lift a trough device 15 and insert a heat resistant sleeve 17 into the opening of the atmosphere protective valve 13. When molten metal is discharged out of the molten metal discharging port 12 of the trough 7 of the melting furnace, the molten metal passes through the heat resistant sleeve 17 and is introduced into a trough 18 for transferring the molten metal, which is formed of a heat resistant material, immediately. The atmosphere protective valve is protected by the heat resistant sleeve whereby trouble will never be happened at the time of passing the molten metal through the atmosphere protective valve and the stability of operation can be secured.