PURPOSE:To wash an electromagnetic filter extremely effectively, by using a liquid containing surface active agent and a compressed air while combining them properly. CONSTITUTION:A washing system, incorporating a washing liquid tank 61 provided with an electric heater 51 as a heater, a liquid feeding pump 12 connected to the tank 61, and a piping provided with valves 27, 28, is connected to an electromagnetic filter 32, to which a filtering system and a backwashing system are connected respectively, so as to form a circulating passage containing said filter 32. Previously, an aqueous solution of surface active agent is prepared in the tank 61, and then is heated. The solution is fed into the filter 32 through the valve 27 by the pump 12, and the liquid after washing is returned to the tank 61 through the valve 28. In this circulating state, a compressed air is intermittently injected into the filter 32 through a valve 29.
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