PURPOSE: To reduce contamination of a steam vessel and a heating device caused by stickness of flux in a paper soldering apparatus and to quicken cooling speed of a material to be treated.

CONSTITUTION: The material to be treated is carried into the steam vessel 1 through a conveyor and heated to execute the soldering treatment. In the case of executing removal of the flux from treating solution 10, the treating solution 10 is dropped into a recovering tank 2 and circulated through a filter 4. The cleaned treating solution is returned back to the steam vessel 1 through a solution supplying tank 3. In order to quicken the cooling speed of the material to be treated, in a condenser (not shown in figure) at outlet side of the steam vessel 1, an injecting hole for coolant in controller pressure and flow rate is provided to cool the material to be treated.