This method for determining whether an acidic aqueous solution is usable or not comprises: a first detection step (S21) in which the solution concentration of an acidic aqueous solution resulting from pickling is determined; a second detection step (S23a, S23b) in which the concentration of a specific metal in the acidic aqueous solution resulting from pickling is determined, the metal having been contained in the coating layer; a concentrated-solution addition step (S25b) in which in cases when the solution concentration is equal to or less than a predetermined first threshold, 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, it is determined that the acidic aqueous solution is unusable.