Provided is a flux for resin-cored solder that is used in resin-cored solder that is supplied into a through hole formed along a central axis of a soldering iron. The flux includes 60% by mass to 99.9% by mass of a rosin ester to a total mass of the flux, 0.1% by mass to 15% by mass of a covalent halogen compound to the total mass of the flux, and more than 0% by mass to 10% by mass of rosin amine, N,N-diethyloctylamine, or rosin amine and N,N-diethyloctylamine to the total mass of the flux.