Target fluid is made into electrolyte solution when measured fluid-quality value is lower than a first condition value, by substituting an impurity anion contained in the target fluid with a predetermined anion and substituting an impurity cation contained in the target fluid with a predetermined cation, and purified when the fluid-quality value is higher than a second condition value. The above procedures are repeated, so that the fluid-quality value of the target fluid falls within a predetermined range, to make the target fluid into electrolyte solution with a correlation between pH and conductivity.