PURPOSE: To enhance dehydration function without bringing about a rise in installation cast by performing a backwashing process while pressing the filter plates of a filter press and reutllizing filtered water to prevent the damage of filter cloth.

CONSTITUTION: A means removing the polluted water at the time of backwashing from the drain water of filter plates 1 and a means gathering filtered water to a filtered water tank 9 are provided and the gathering pipes 4,15 connected to the filtered water tank 9 are provided to be connected to the filter plates 1 of respective odd number-th and even number-th blocks by flexible tubes and filtered water is allowed to flow back at every blocks and introduced into the filter plates 1 to backwash the filter plates 1 and the damage and expansion of the filter cloths of the filter plates 1 are prevented.