To provide an inexpensive and simple water treatment method and a device therefor, which can be used for a wide range of wastewater and in which the generation of sludge is small and energy consumption is small.

In the water treatment method, basic substance 6 such as sodium hydroxide is added to wastewater 1 to adjust pH to 10 or higher, and bivalent iron ions 5 are then added thereto to precipitate and remove pollution particles such as organic substance, inorganic substance such as heavy metals, ions, and suspended matter. By adding amorphous ferric hydroxide colloid particles having a diameter of 30 nm or less to supernatant liquid, remaining small suspended particles or anions are precipitated and removed.