Method and apparatus for producing pure water

A method for producing ultrapure water using an ultrapure water production facility including, a primary pure water production device, and a secondary pure water production device whereby ultrapure water produced in the secondary pure water production device is fed to a point-of-use, excess ultrapure water from the point-of-use is returned and mixed with primary pure water from the primary pure water production device to produce mixed water, and the mixed water is treated as water-to-be-treated by the secondary pure water production device,

• wherein in the secondary pure water production device, the water-to-be-treated is subjected to an ultraviolet oxidation treatment performed with an ultraviolet oxidation device and subsequently subjected to a hydrogen peroxide removal treatment performed with a hydrogen peroxide removal device including a platinum-group metal catalyst,
• wherein the TOC concentration in water fed to the ultraviolet oxidation device is 5 ppb or less, and
• wherein the concentration of inorganic carbonate ions in the water fed to the ultraviolet oxidation device is less than 1 ppb, and the concentration of inorganic carbonate ions in water that has been subjected to the ultraviolet oxidation treatment performed with the ultraviolet oxidation device is 1 ppb or more.