Method for manufacturing soldered products

A method for manufacturing a soldered product by soldering a solder object portion, comprising:

• a solder supplying step that causes a cylindrical soldering iron having a through hole to contact with the solder object portion to supply a thread solder piece to the solder object portion from the through hole;
• a heating step that heats the thread solder piece with the cylindrical soldering iron and causes to melt the thread solder piece at the solder object portion;
• and
• a curing step that cures a melting object of the thread solder piece to solder the solder object portion,
• wherein
• the thread solder piece is composed of a core containing a flux and a coating member containing a solder alloy that covers the core,
• the flux has a rosin having an acid value of less than 10 KOHmg/g as a component thereof, and
• an amount of heating loss of the flux at 400° C. is smaller than or equal to 55 mass %.