A tin or solder alloy film is formed by ejecting molten tin liquid or molten solder alloy liquid in a jet state onto a copper surface of an electronic component. The overflowing molten tin or molten solder alloy liquid is transported to a stirrer. Then, cuprate and the like mixed in the overflowing molten tin or molten solder alloy liquid are taken in to the organic fatty acid solution by fiercely stirring and mixing the molten tin or molten solder alloy liquid in contact with a solution at a temperature of 180 to 350° C. containing 1 to 80 weight % of organic fatty acid having 12 to 20 carbon number that circulates in the stirrer to purify the overflowing molten tin or molten solder alloy liquid. The mixed solution is introduced into a reservoir of the organic fatty acid solution, and the purified molten tin solution or molten solder alloy liquid separated according to the specific gravity difference in the reservoir of the organic fatty acid solution is returned from the bottom of the organic fatty acid solution reservoir to the reservoir of the molten tin or molten solder liquid by a pump for circulation use. By means of this, copper is prevented from accumulating in the molten tin or molten solder liquid, and thus it is possible to continuously perform a stable soldering process for a long term.