[Problems] Addition of an organic acid such as a dicarboxylic acid to a soldering flux in order to improve wettability causes a reaction with copper oxide to form a metallic soap of copper having a green color. Although this metallic soap of copper is not corrosive and does not decrease reliability, its external appearance is impossible to distinguish it from verdigris which is indicative of corrosion. There is need for a soldering flux which does not form a metallic soap of copper.[Means for Solving the Problems] Addition of a tetrazole or a tetrazole derivative to a soldering flux can prevent carboxyl groups from reacting exclusively with copper ions, thereby suppressing the formation of a metallic soap of copper having a green color. Among tetrazoles and tetrazole derivatives, 5-phenyl-1H-tetrazole and its derivatives which have hydrogen as a substituent in the 1-position and a phenyl, group which strongly attracts electron in the 5-position are effective at improving soldering since they result in an increased polarity in the solvent.