The amounts of impurity metal ions incorporated into electrolytic manganese dioxide (EMD) during its preparation were measured as a function of metal ion concentrations and current densities. The amount of incorporated ions increased in proportion to the concentration in solution, and at a fixed concentration it was different from ion to ion : Ni2+ < Zn2+ < Co2+ < Cu2+ < Fe3+ < Pb2+. The specific surface area of the formed EMD was larger for impurity ions with higher incorporation affinity. Further, the adsorption of ions on the surface of a ready-made manganese dioxide sample (IC12) was examined, and modeling of the adsorption behavior was attempted. The amounts of adsorbed ions at a fixed concentration in solution and pH 0.7 (where EMD is produced) were obtained by the ion-adsorption model. There was a strong correlation between the amount incorporated and the amount of adsorption, suggesting a mechanism in which EMD is contaminated through adsorption on its new growing surface. The increase in specific surface area of EMD with contaminants was interpreted to be due to a suppression of the growth of EMD at the adsorbed foreign ion sites, resulting in EMD with many defects or smaller particle sizes. The opposite effect of current density on incorporation for the two groups of metal ions was discussed.