PURPOSE: To prevent temperature control impossibility and to obtain stable production activity by circulating nonoxidizing gas in a soldering furnace between the furnace and a cleaning column via a circulating duct.

CONSTITUTION: The nonoxidizing gas in the soldering furnace 3 is sucked into the cleaning column 11 via the circulating duct 12 by an action of a fan 19 for circulation. The sucked nonoxidizing gas rises in the cleaning column 11 and moisture and a flux component which are rapidly cooled and vaporized at the time of passing a heat exchanger 14 are condensed and liquefied, and stuck to the heat exchanger 14. A stuck residum is mixed or dissolved in water flowing out from water showers 16 and mixed in water in a vessel 22 from a drain port 21. The nonoxidizing gas where the residue is removed as again cooled by a heat exchanger 18 for removing the moisture, after the cooled and condensed gas is further separated by mist showers 17, and steam held by the mist showers 17 is condensed and removed. The nonoxidizing gas where the moisture and flux are removed is again filled into the soldering furnace 3.