Abstract: Ultrapure water having a reduced amount of TOC and a low electrical conductivity is produced in correspondence with a variation in water quality of raw water. A first water tank has an anaerobic organic matter treatment chamber and an aerobic organic matter treatment chamber which are communicated with each other at their bottom portions. Activated carbon bags are arranged in upper portions of both the chambers, and Bincho charcoal is stowed in the lower portion of the chambers. With this arrangement, carbon compounds and organic nitrogen compounds in the water to be treated are biologically treated by microbes, thereby reducing the TOC. Furthermore, nitrate nitrogen generated through the organic nitrogen compound treatment process is denitrified by the anaerobic microbe, thereby reducing the electrical conductivity of the water to be treated.