The behaviors of manganese ions during biological precipitation of iron ions by Acidithiobacillus ferrooxidans was investigated to remove the Mn ions from acid mine drainage. The effect of initial $Fe^{2+}$ ion concentration with $900g{cdot}m^{-3}$, $4500g{cdot}m^{-3}$, and $9000g{cdot}m^{-3}$ on the removal of Mn ions was examined by measuring pH, ORP, and the concentrations of Fe and Mn ions. In the experiments with the $Fe^{2+}$ concentration with $4500g{cdot}m^{-3}$ and $9000g{cdot}m^{-3}$, pH increased and then decreased by the oxidation of $Fe^{2+}$ and the precipitation of Fe ions. In all solutions, ORP increased to 630 mV, and Fe concentration decreased with Fe precipitation. Adding NaOH increased the amount of Fe precipitate and the Mn removal ratio in the early stage but there was no significant effect as the experiments were proceeded.