To reduce the concentration of NH3 to be treated along with a high pH operation remarkably compared to the conventional apparatus to evade the increase of the capacity of a desalting tower.

This gas permeation membrane type pre-treating apparatus is arranged in the upstream side of the desalting tower which is one component of a pressurized water nuclear power plant or a thermal electric power plant to be used, has a casing 12 and a hydrophobic membrane 15 for separating the casing 12 into an upstream side chamber 13 and a downstream side chamber 14 and non-permeable to impurities in water and is constituted so that the upstream side chamber 13 and the downstream side chamber 14 are connected respectively to an inlet part of the desalting tower 18 and to an outlet part of the desalting tower 18, a condensate having high ammonia concentration is introduced into the upstream side chamber 13 and the treated water through the desalting tower 18 is introduced into the downstream side chamber 14 from the outlet part of the desalting tower 18.