PURPOSE: To recover and regenerate a filtration function easily by removing simply an inclusion which adheres to and is retained in a passage from the filter surface on the side of a front flow chamber of a porous filter member to the inside of each pore for filtration without arranging especially the constitution of a pipeline to obtain a back washing effect.

CONSTITUTION: When an inclusion which adheres to and is retained in a porous filter member 12 associated with the filtration action is removed, the supply of pressurized fluid (a) to be filtered to a front flow chamber 13 is stopped, and also the removal of filtrate (b) in a rear flow chamber 14 is stopped. Then, a front flow chamber 13 side is opened to a lower pressure side so that the inclusion can be removed by the pressure difference between the pressure of the filtrate (b) left in the rear flow chamber 14 and the pressure due to the opening of the lower pressure side.