To solve the problem in a conventional water treatment using a metal electrode that the effective electrode area is reduced, when the scale component quantity is large, by precipitation of metal impurities on the metal electrode surface, and a long-term stable operation cannot be performed.

In this water treatment system, the scale component quantity of water 7 to be treated, which is supplied to the electrochemical water treatment device 1 having the metal electrode 5, is retained at 250 mg/kg or less in terms of CaCO3. According to this, the quantity of the metal impurities precipitated on the metal electrode surface can be significantly reduced, and the electrochemical water treatment of water in a cooling tower can be performed without substantially stopping the operation.