The concentration of Hydrogen Peroxide (or Sodium Chlorite) in the bleaching bath is constantly measured by means of light absorbing optical method. According to the measured value, the required volume of thicker fresh Hydrogen Peroxide liquid (or Sodium Chlorite) is measured and dosed continuously in to the bath to keep the concentration constantly at preset value which is registered in the finishing program file. The 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 does is determined by the calculation which is based on the proportion to the dosing volume of Hydrogen Peroxide (or Sodium Chlorite) pH of the bleaching liquid is continuously monitored by high sensitive pH sensor. pH is kept constant by dosing required volume of NaOH liquid.