This invention relates to a method for preparing an acid addition salt of a polyacidic basic compound, or a water adduct having basic site(s) stronger than pyridine. The method comprises reacting the polyacidic basic compound with an acid salt of pyridine.By the present invention, the number of moles of an added acid in the acid addition salt of the polyacidic basic compound can be readily changed to a number suited for the polyacidic basic compound as needed.