PURPOSE: To prevent the oxidation of molten solder by supplying a liquid antioxidant or inert gas to the liquid surface of the molten solder around a revolving shaft enclosed by a hollow body.

CONSTITUTION: The cylindrical hollow body 8 formed with an injection hole on the flank which is hermetically closed at its top end exclusive of a hole for passing the revolving shaft is provided to cover the part where the revolving shaft 5 emerges from the molten solder 2 vertically around this part. The liquid antioxidant 9, for example, silicone oil or peanut oil, is injected and packed from the injection hole of the hollow body 8 onto the liquid surface 2a of the molten solder 2 around the revolving shaft 5 enclosed by the hollow body 8. As a result, the contact of the molten solder 2 with air in the hollow body 8 is averted and, therefore, the oxidation of the molten solder on the liquid surface of the molten solder 2 around the revolving shaft 5 is substantially entirely obviated.