A method for producing solutions, the method using a sulfuric acid solution containing nickel, cobalt, and calcium and performing the following steps in parallel: a first step for producing a mixed solution of nickel sulfate and cobalt sulfate from the sulfuric acid solution; and a second step for producing a solution of nickel sulfate from the sulfuric acid solution, wherein in the first step, the sulfuric acid solution is subjected to solvent extraction by means of an extractant to obtain a first organic solvent after extraction containing calcium and a first extraction residue containing nickel and cobalt, and in the second step, the sulfuric acid solution is subjected to solvent extraction by means of an extractant to obtain a second organic solvent after extraction containing cobalt and calcium and a second extraction residue containing nickel, wherein the second step includes: an extraction step for subjecting the sulfuric acid solution to solvent extraction by means of an extractant to obtain a second organic solvent after extraction containing cobalt and calcium and a second extraction residue containing nickel; and a stripping step for subjecting the second organic solvent after extraction to stripping to obtain an organic solvent after stripping containing calcium and a stripping liquid containing cobalt, and in the first step, an extraction starting liquid obtained by mixing the stripping liquid obtained in the second step with the sulfuric acid solution at a predetermined ratio is subjected to solvent extraction by means of the extractant. a first step for producing a mixed solution of nickel sulfate and cobalt sulfate from the sulfuric acid solution; and a second step for producing a solution of nickel sulfate from the sulfuric acid solution, wherein in the first step, the sulfuric acid solution is subjected to solvent extraction by means of an extractant to obtain a first organic solvent after extraction containing calcium and a first extraction residue containing nickel and cobalt, and in the second step, the sulfuric acid solution is subjected to solvent extraction by means of an extractant to obtain a second organic solvent after extraction containing cobalt and calcium and a second extraction residue containing nickel, wherein the second step includes: an extraction step for subjecting the sulfuric acid solution to solvent extraction by means of an extractant to obtain a second organic solvent after extraction containing cobalt and calcium and a second extraction residue containing nickel; and a stripping step for subjecting the second organic solvent after extraction to stripping to obtain an organic solvent after stripping containing calcium and a stripping liquid containing cobalt, and in the first step, an extraction starting liquid obtained by mixing the stripping liquid obtained in the second step with the sulfuric acid solution at a predetermined ratio is subjected to solvent extraction by means of the extractant. an extraction step for subjecting the sulfuric acid solution to solvent extraction by means of an extractant to obtain a second organic solvent after extraction containing cobalt and calcium and a second extraction residue containing nickel; and a stripping step for subjecting the second organic solvent after extraction to stripping to obtain an organic solvent after stripping containing calcium and a stripping liquid containing cobalt, and in the first step, an extraction starting liquid obtained by mixing the stripping liquid obtained in the second step with the sulfuric acid solution at a predetermined ratio is subjected to solvent extraction by means of the extractant.