A filtration filter comprises a porous film including a central film portion having a plurality of through holes and an outer edge portion adjacent to the central film portion. The porous film portion lies in a flat plane in the absence of a force being applied to the central portion of the central film portion. A frame holds the outer edge portion of the porous film in such a manner that when a force is applied to the central film portion in a first direction, the central film portion moves in the first direction relative to the flat plane and the outer edge portion moves in a second direction, opposite to the first direction, relative to the flat plane. In this way stresses applied to the porous film during a filtering operation are reduced.