A method for determining whether acid-containing aqueous solution is useful

A first detection step (S21) in which the solution concentration of an acidic aqueous solution resulting from the pickling, it is determined a second detection step (S23a, S23b): This method for determining whether an acid-containing aqueous solution is useful or not, comprising in which the concentration of a specific metal in the acidic aqueous solution resulting from the pickling, it is determined, wherein the metal contained in the coating layer; an addition step of a concentrated solution (S25b), in which, in cases where the solution concentration is equal to or less than a predetermined first threshold value, a concentrated acidic aqueous solution is newly added and a determination step (S24a, S24b) in which, in cases when the concentration of the specific metal is equal to or higher than a predetermined second threshold value, determines that the acidic aqueous solution is unusable.