A solution containing an organic solvent and a compound including impurities, represented by, for example, general formula (1) (wherein R1 to R10 independently represent H, F, a hydrocarbon group or an alkoxy group, at least one of R1 to R5 is F, at least one of R6 to R10 is F, M is H, alkali metal, alkaline earth metal or alkaline earth metal halide, n is 3 or 4, and m is 2 when M is an alkaline earth metal, or m is 1 when M is the other) is mixed with water. The purified compound is reacted with a compound which forms a monovalent cation seed to prepare a derivative. Thus, a purifying method capable of efficiently and easily separating and removing colored components contained in tetrakis(fluoroaryl)borate compounds can be provided. Moreover, it is possible to provide a method capable of preparing a highly-pure tetrakis(fluoroaryl)borate derivative efficiently and at low costs.