To provide a water treatment system in which it is not necessary to install a measurement tank or an electromagnetic flowmeter, entire system costs can be reduced, water can be sent from a flow rate adjusting tank to a treatment tank without waste, and an energy saving effect can be obtained.

A flow rate adjusting tank 3 is arranged on a stage before a treatment tank 2 for performing water treatment, and the flow rate adjusting tank 3 includes: a pump 5 for transporting water to be treated to the treatment tank 2; and water level detection means SW1-SW4. On the other hand, there is provided a control means 7 which controls operation of the pump 5 on the basis of detection output of the water level detection means SW1-SW4, and the control means 7 determines a total pump head H under a required discharge quantity Qreq in accordance with a water level of the flow rate adjusting tank 3 detected by the water level detection means SW1-SW4. In accordance with a magnitude of the total pump head H, an operation performance of the pump 5 is varied and water to be treated for the required discharge quantity Qreq meeting the throughput of the treatment tank 2 is transported to the treatment tank 2.