The filter medium for a filter of the present invention has a laminate structure in which a long-fiber non-woven fabric and a wet non-woven fabric are stacked, wherein the wet non-woven fabric comprises vinylon, polyester, and polyvinyl alcohol, is disposed on a most upstream side of air flow, has a surface located on the most upstream side of air flow, the surface having a surface roughness (SMD) not larger than 2.7 μm, and has a surface stacked on the long-fiber non-woven fabric, the surface having (a) a surface roughness (SMD) not smaller than 3.0 μm or (b) a rough texture (MMD) not lower than 0.02.