To provide a water treatment system allowing simplification of the system and a reduction in running cost.

The water treatment system includes: a first flow line L1 for allowing ground water W1 to flow in an anaerobic condition; a cation exchange device 2 for producing treated water W2 by introducing the ground water W1 into a cation exchanger 3; a second flow line L2 for allowing the treated water W2 to flow in an anaerobic condition; a treated water storage tank 6 for storing the treated water W2 in an anaerobic condition; and a third flow line L3 for allowing the treated water W2 to flow in an anaerobic condition to a water-supplied device 7. In the ground water W1 and the treated water W2, pH values are maintained in a range of 5 to 9, and oxidation-reduction potentials are maintained in a range of E1 to E2, where E1 and E2 are determined by equations: E1[V]=-0.059×(pH value) and E2[V]=0.7-0.059×(pH value).

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