Adsorption properties of divalent and trivalent metal ions onto sodium polyacrylate gel are investigated over a range of pH of buffer solutions. It is considered that carboxyl groups in the gel combine with the metal ions according to their charge number stoichiometrically, and the adsorption mechanism is theoretically analyzed on the basis of dissociation equilibrium for monocarboxyl acid. Furthermore, it is found that the adsorption effectiveness factor, being intrinsic to metal ion species, fulfills an important role in order to elucidate the adsorption equilibrium. Consequently, the model equations, which allow quantitative consideration of the adsorption mechanism in these systems, can be derived as a function of the hydrogen ion concentration, which can readily be measured in the outer aqueous solution. The gel adsorption equilibrium of divalent and trivalent metal ions can be explained fairly well by the model.