A method for producing high-purity nickel sulfate, the method comprising a production process for producing high-purity nickel sulfate from a solution containing nickel by a solvent extraction step, and the method subjecting a nickel sulfate solution with a high concentration of impurities obtained in the production process to a removal treatment of an impurity element of magnesium through the following steps to in order, to discharge the impurity element of magnesium included in the nickel sulfate solution with the high concentration of impurities out of the production process so as to prevent accumulation of the impurity element of magnesium in the production process, thereby forming high-purity nickel sulfate having a reduced amount of the impurity element of magnesium: a hydroxylation step of adding an alkali hydroxide to a portion of the nickel sulfate solution with the high concentration of impurities in the production process, with a pH range of 7.5 to 7.8, to form a hydroxylated slurry including a precipitate of nickel hydroxide and a solution after hydroxylation other than the precipitate, the precipitate of nickel hydroxide being caused by precipitation of the nickel contained in the solution; a carbonation step of adding an alkali carbonate to the hydroxylated slurry, with a pH range of 7.7 to 8.0, and subjecting the nickel contained in the solution after hydroxylation to a carbonation treatment to form a carbonated slurry including a precipitate of nickel carbonate and a solution after carbonation other than the precipitate of nickel carbonate, the precipitate of nickel carbonate being caused by precipitation of the nickel contained in the solution after hydroxylation, and then forming a mixed slurry including a mixed precipitate of the nickel hydroxide and the nickel carbonate, and a solution after reaction other than the mixed precipitate, the mixed precipitate being formed by converting the nickel included in the nickel sulfate solution with the high concentration of impurities; a solid-liquid separation step of separating the mixed slurry into the mixed precipitate of nickel hydroxide and nickel carbonate, and the solution after reaction, the mixed precipitate being used as a neutralizing agent for use in pH adjustment prior to the solvent extraction step; and a neutralization step of subjecting the solution after reaction to a neutralization treatment in a pH range of 8.0 to 8.5 to produce a neutralized solution that includes a solution after neutralization and a neutralized precipitate containing an impurity element of magnesium, and subjecting the neutralized solution to a solid-liquid separation, and then recovering the impurity element of magnesium included in the solution in the production process as the neutralized precipitate containing the impurity element of magnesium. a hydroxylation step of adding an alkali hydroxide to a portion of the nickel sulfate solution with the high concentration of impurities in the production process, with a pH range of 7.5 to 7.8, to form a hydroxylated slurry including a precipitate of nickel hydroxide and a solution after hydroxylation other than the precipitate, the precipitate of nickel hydroxide being caused by precipitation of the nickel contained in the solution; a carbonation step of adding an alkali carbonate to the hydroxylated slurry, with a pH range of 7.7 to 8.0, and subjecting the nickel contained in the solution after hydroxylation to a carbonation treatment to form a carbonated slurry including a precipitate of nickel carbonate and a solution after carbonation other than the precipitate of nickel carbonate, the precipitate of nickel carbonate being caused by precipitation of the nickel contained in the solution after hydroxylation, and then forming a mixed slurry including a mixed precipitate of the nickel hydroxide and the nickel carbonate, and a solution after reaction other than the mixed precipitate, the mixed precipitate being formed by converting the nickel included in the nickel sulfate solution with the high concentration of impurities; a solid-liquid separation step of separating the mixed slurry into the mixed precipitate of nickel hydroxide and nickel carbonate, and the solution after reaction, the mixed precipitate being used as a neutralizing agent for use in pH adjustment prior to the solvent extraction step; and a neutralization step of subjecting the solution after reaction to a neutralization treatment in a pH range of 8.0 to 8.5 to produce a neutralized solution that includes a solution after neutralization and a neutralized precipitate containing an impurity element of magnesium, and subjecting the neutralized solution to a solid-liquid separation, and then recovering the impurity element of magnesium included in the solution in the production process as the neutralized precipitate containing the impurity element of magnesium.