A method comprising neutralizing and removing a metal ion remained in a barren solution produced through a sulfurization treatment for producing a sulfide containing nickel and cobalt by blowing a hydrogen sulfide gas into a sulfuric acid aqueous solution containing the nickel and the cobalt, and impurity metals containing at least one or more of iron, magnesium, and manganese, the method comprises: S1) a leaching step of adding sulfuric acid to a nickel oxide ore slurry, the ore comprising chiefly laterite ore, under a temperature of 220 to 280°C under high pressure to produce leached slurry composed of leachate and a leached residue; S3) a neutralization step of producing a mother liquid for nickel recovery composed of a sulfuric acid aqueous solution containing nickel, cobalt and impurity metals containing at least one or more of iron, magnesium, and manganese and removing trivalent ferrous ion as sediment slurry by adding calcium carbonate to the leachate produced in step S1 so that pH of the leachate is 4.0 or less while suppressing oxidation of the leachate; S4) a sulfurization step of producing a barren solution and a sulfide containing nickel and cobalt by blowing a hydrogen sulfide gas into the mother liquid produced in step S3; S11) a first neutralization treatment step of applying a neutralization treatment to the barren solution produced in step S4, with a range of pH 5.0 to 6.0 as an end point, by using calcium carbonate slurry as a first neutralizer; and S12) a second neutralization treatment step of applying a neutralization treatment to a solution produced in the first neutralization treatment step by using slaked lime slurry as a second neutralizer to produce a sediment containing a metal remained in the barren solution and a post-neutralized solution from which the metal ion is removed. S1) a leaching step of adding sulfuric acid to a nickel oxide ore slurry, the ore comprising chiefly laterite ore, under a temperature of 220 to 280°C under high pressure to produce leached slurry composed of leachate and a leached residue; S3) a neutralization step of producing a mother liquid for nickel recovery composed of a sulfuric acid aqueous solution containing nickel, cobalt and impurity metals containing at least one or more of iron, magnesium, and manganese and removing trivalent ferrous ion as sediment slurry by adding calcium carbonate to the leachate produced in step S1 so that pH of the leachate is 4.0 or less while suppressing oxidation of the leachate; S4) a sulfurization step of producing a barren solution and a sulfide containing nickel and cobalt by blowing a hydrogen sulfide gas into the mother liquid produced in step S3; S11) a first neutralization treatment step of applying a neutralization treatment to the barren solution produced in step S4, with a range of pH 5.0 to 6.0 as an end point, by using calcium carbonate slurry as a first neutralizer; and S12) a second neutralization treatment step of applying a neutralization treatment to a solution produced in the first neutralization treatment step by using slaked lime slurry as a second neutralizer to produce a sediment containing a metal remained in the barren solution and a post-neutralized solution from which the metal ion is removed.