To quantify cations contained in a strong acidic solution or anions contained in a strong basic solution by capillary electrophoresis or ion chromatography.

After the strong acidic solution is brought into contact with an OH type anion exchange resin and pH of the solution is increased to 4 or higher, the cations contained in the solution are quantified by the capillary electrophoresis or ion chromatography. Also, after the strong basic solution is brought into contact with an H type cation exchange resin and the pH of the solution is reduced to 10 or below, the anions contained in the solution is quantified by the capillary electrophoresis or ion chromatography.