PURPOSE: To prevent the deterioration in filtering capacity caused by long-term continuous use by further increasing the washing effect of an electromagnetic filter, by combining the regular washing process of filter media with the backwashing process thereof.

CONSTITUTION: Only a raw water introducing valve 7, a filtrate discharge valve 10 and an opening and closing valve 16 are opened and raw water is supplied to an electromagnetic filter 1 by a pump 6 and magnetic particles in raw water are collected by magnetized filter media. A definite amount of the filtrate is stored in a storage tank 9 and the excessive part is discharged from the valve 10. In washing the media 3, the magnetization thereof is released and only an air valve 3, the valve 16 and a washing water discharge valve 8 are opened while high pressure air of an air tank 11 is introduced into the tank 9 and sends the washing water in the tank 9 to the filter 1 from a secondary side 5 under pressure to perform backwashing. Herein, only the valve 13, a discharge valve 15 and a regular washing water supply valve 18 are further opened and the washing water in the tank 19 is passed through an arranged pipe 17 and guided to the filter 1 and a discharge pipe 14 from a primary side 4 to perform regular washing.