The present invention provides a cation exchange membrane which has excellent durability, a high limiting current density, a low direct current membrane resistance and excellent selectivity to monovalent cations. The support member is supported by the movable support member and the movable support member is supported by the support structure. The rear pole plate 14 and the impedance converter 43 are accommodated in a metal case 52. Thus, until Tr completes charging up C, a duty ratio of a pulse width modulation signal dependent on the SM output is made gradually greater, thereby surely preventing a rush current by turning on and off the FET by the PWM signal. By detecting the presence of liquid rather than gas, false alarms due to micro bubbles or small air gaps between the sensor and the tubing are avoided.