outer rim 6 throughout the circumference of the filtration membrane unit so that the filtration membranes 2 are sealed the overlap width margin to paste up in the direction of radius in the inner circumferential edge portions 11 and in the outer circumferential edge portions 12 are not limited specifically but it is preferable that the overlap width is determined in consideration for liquid tightness reduction of effective filtration area due to sticking and so on in this embodiment the range of 0 1 to 5 mm is suitable for the overlap width the membrane support 14 formed of paper nonwoven fabric or netty material is provided to support the pair of upper and lower filtration membranes 2 from the inside but is not stuck thereto accordingly the filtration membranes can be increased in effective filtration area and can bear up against relatively high filtration pressure furthermore the process of assembling the filtration units can be simplified it is a matter of course that the membrane support 14 may be stuck to the inner and outer rims 3 and 6 in the present invention a membrane support 14 formed of paper nonwoven fabric or netty material is interposed between the pair of upper and lower filtration membranes 2 the nonwoven fabric or netty material is not limited specifically but it is preferable that polyester or polypropylene is used as the material and a long staple fiber is used for preparing the material in this viewpoint a span bond production method is used in the present invention further it is preferable that the membrane support of nonwoven fabric or netty material is not stuck to the filtration membranes although means of sealing and.