A composite metal polybasic salt containing a trivalent metal and magnesium as metal components and having a novel crystal structure, and a method of preparing the same. The invention further deals with a composite metal polybasic salt which has anion-exchanging property, which by itself is useful as an anion-exchanger, capable of introducing anions suited for the use upon anion-exchange, and finds a wide range of applications, and a method of preparing the same. The composite metal polybasic salt has a particular chemical composition and X-ray diffraction peaks, and further has a degree of orientation (I o ) of not smaller than 1.5.