The concentration of NaOH solution in the finishing bath is measured continuously by the electro magnetic detector. Following the concentration deviation, required volume of fresh NaOH is measured and dosed continuously in to the finishing bath to keep the concentration constantly at preset value which is prescribed in the finishing program file. Required volume of the auxiliaries are also measured and dosed in to the bath to keep the concentration constantly at preset value prescribed in the finishing program file. The volume of the auxiliaries to be dosed is determined by the calculation which is based on the proportion to the dosing volume of NaOH.