Provided is a filtration filer with which a stable opening size during filtration is obtained, and which can be easily regenerated when clogged and used repeatedly. The filtration filter (10) comprises: a substrate stack wherein substrates (11) that have a through hole that penetrates from front to back are stacked; stoppers (15) that define the spacing between the substrates (11); and pillars (16) with a larger thermal expansion coefficient than the stoppers (15). The spacing between the substrates (11) at normal temperature is defined at least by the stoppers (15), and the spacing when heated is defined by the thermally expanded pillars (16). The gaps between the substrates (11) form the filtration surface that traps contaminants contained in the liquid that is being treated A.