PURPOSE: To enhance recoverability by adding sewage sludge incinerated ash to org. waste water to form magnetic floc.

CONSTITUTION: The raw water flowing in a main body 3 from a raw water inflow part 1 passes through a magnetized matrix 4 and, during this period, the magnetic fine particles in the raw water are attracted to the matrix 4 by magnetic force and treated water flows out from a treated water outflow part 2. When continuous treatment is performed, the magnetic fine particles are accumulated on the matrix 4 and the magnetic fine particle attraction capacity of the matrix 4 is reduced. In order to prevent this phenomenon, the backwashing of the matrix 4 is periodically performed. As a magnetic powder added to org. waste water, sewage sludge incinerated ash is used. By this method, the cost of chemicals can be reduced to a large extent.