Continuous Electrodeionization (CEDI) is beginning to be used widely to water treatment systems for the microelectronics, pharmaceutical and power generation industries. The recent microelectronics field requires ultrapure water (UPW) at very low concentration of silica and boron. Whereas, it is difficult to remove such weakly ionized substances up to the required level by conventional CEDI. We have developed a new CEDI, which makes it possible to produce high quality water with silica and boron concentrations below sub ppb level. This paper provides the mechanism of this new technology and performance characteristics of the CEDI module using this technology.