Abstract: A regeneration method to rapidly and efficiently desorb ions after the ions are absorbed to electrodes in a deionization apparatus to eliminate ion components in a fluid (liquid and gas) is disclosed. A plurality of cells including a plurality of electrodes to absorb ions included in a fluid are connected to configure a stack. In a capacitive deionization (CDI) apparatus including at least two stacks, if 0 V is applied as a method of desorbing the ions and regenerating the electrodes after the ions are absorbed to the electrodes, and the cells or the stacks are connected in series in a state in which the cell units and the stack units obtained by connecting the cells are electrically disconnected from a power source, the capacitance of the entire system is reduced, a discharging time is shortened, and the ions are rapidly and efficiently desorbed.