A filter includes a plurality of filter units formed of a porous ceramic medium arranged and housed in a casing, and having therethrough a circulatory line for passing a feed fluid and a discharging route for passing a permeated fluid. The circulatory line and the discharging route are separated from each other by partially sealing those portions of the filter units which are fixed to opposite ends of the casing. The filter unit is structured to basically have a large number of open channels communicating with the circulatory line and passing the feed fluid and closed channels passing the permeated fluid, and a large number of discharge holes penetrating a solid portion of the porous ceramic medium to discharge the permeated fluid. As seen in any cross section of the filter orthogonal to one direction thereof, the plurality of filter units occupy an area corresponding to at least 35% of an internal area of the casing. Such structure can provide an increased permeated flow while the fluid therethrough with small pressure drop.