According to one embodiment, a water treatment system includes: a membrane separation device and a water softening device including a valve unit switching between a softening process in which soft water is obtained by passing raw water through a cation exchange resin bed downward and a regeneration process in which a whole of the cation exchange resin bed is regenerated by generating an opposite flow of a regenerant by supplying the regenerant from both sides of a top portion and a bottom portion of the cation exchange resin bed and collecting a liquid at an intermediate portion of the cation exchange resin bed, and a regenerant supply supplying the regenerant in a volume which gives a regeneration level of 1 to 6 eq/L-R, to a hardness leakage prevention region set in a predetermined depth on the bottom portion of the cation exchange resin bed, in the regeneration process.