a step A of performing an ion exchange method on a substance to be purified, which includes one or more kinds of periodic acids selected from the group consisting of a periodic acid and a salt thereof, one or more kinds of first metal components selected from the group consisting of Ti and Zr, and water, so as to obtain the chemical solution,

wherein the chemical solution comprises:

one or more kinds of periodic acids selected from the group consisting of a periodic acid and a salt thereof;

one or more kinds of first metal components selected from the group consisting of Ti and Zr; and

water,

in a case where the chemical solution includes one kind of first metal component, a content of the one kind of first metal component is 1 ppt by mass to 100 ppm by mass with respect to a total mass of the periodic acids in the chemical solution, and

in a case where the chemical solution includes two kinds of first metal components, a content of both the two kinds of first metal components is equal to or smaller than 100 ppm by mass with respect to the total mass of the periodic acids in the chemical solution, and a content of at least one of the two kinds of first metal components is equal to or greater than 1 ppt by mass with respect to the total mass of the periodic acids in the chemical solution, and

wherein the pH of the chemical solution is 6.8 to 10.0.