PURPOSE: To obtain a filter medium high in filtration performance, lightweight, compact and capable of sterilization by repeatingly or randomly allowing a resin or a metallic material, used as a material and formed into a thin belt-like piece having a prescribed width, to meander.

CONSTITUTION: The filter medium composed of the resin or the metal material is formed into the thin elongated belt like piece. The belt-like piece is corrugated to form curved pieces 1, 1 with a fixed space and is rounded helically as a whole. The spaces 3, 3 remaining in each of the curved pieces 1 are wide in the outer circumference and narrower towards the center and communicate with the center holes each other. When water passes through the filter medium obtained in this way, the flow of water is restrained by each of the curved pieces 1, 1 and the impurities in the water is stuck to the surface of the curved pieces 1, 1.