A flux for soldering used when soldering is performed to a board subjected to electroless nickel plating, the flux containing resin having film forming ability, activator, and solvent, and further containing metallic salt in an amount of 0.1 to 20% by weight of the total amount of flux. The use of this flux suppresses diffusion of nickel solder in soldering portions, and prevents concentration of phosphorous, thereby improving the bonding strength of soldering.