A principal object of the present invention is to provide a flux which can be used to produce a lead-free solder paste which is excellent in viscosity stability and exhibits excellent wettability at the time of soldering even in atmospheric air. The flux is a lead-free solder flux having a bromine atom concentration of 400 to 20000 ppm based on 0.1 g of the flux and comprising 0.01 to 0.7% by weight of an amine compound (a) represented by the general formula (1): H2N—(CH2)n—X—(CH2)n—NH2 (wherein n represents an integer of 1 to 6 and X represents —NH—CH2CH2—NH— or a piperazine residue).