An apparatus for controlling the resistivity of ultra pure water, including a
housing to house a gas permeable membrane, an inlet for untreated ultra pure water
which communicates with an ultra pure water path, and an outlet for resistivity-controlled
ultra pure water which communicates with the ultra pure water path. The gas permeable
membrane divides the interior of the housing into the ultra pure water path and
a mixed gas path. A mixed gas is selected from the group consisting of a mixed
gas including carbon dioxide and a gas having a lower resistivity controlling ability
than carbon dioxide and a mixed gas including ammonia and a gas having a lower
resistivity controlling ability than ammonia. The gas permeable membrane is capable
of supplying carbon dioxide or ammonia to the untreated ultra pure water at a concentration
equal to or more than 90% of the equilibrium concentration.
