A first precipitate is formed by mixing graphite and an oxidizer containing an alkali metal salt in a solution. Next, a second precipitate is formed by ionizing the oxidizer which is included in the first precipitate, with an acid solution, and removing the oxidizer from the first precipitate. Then, a dispersion liquid in which graphene oxide is dispersed is prepared by mixing the second precipitate and water to form a mixed solution and then applying ultrasonic waves to the mixed solution or mechanically stirring the mixed solution, so that the graphene oxide is separated from graphite oxide that is the graphite which is included in the second precipitate and oxidized. Next, graphene oxide salt is formed by mixing the dispersion liquid, a basic solution, and an organic solvent and reacting the graphene oxide included in the dispersion liquid and a base included in the basic solution to each other.