PURPOSE: To practically eliminate the outflow of impurities when a filter cartridge is initially set and to enhance safety by previously finishing hydrophilicity impartation to reduce the startup time after a filter element is set and previously finishing cleaning.

CONSTITUTION: A microphorous thin film 1 having 0.01-5.0μ maximum pore diameter is wetted with an org. solvent compatible with water in the filter element 3 consisting of the thin film 1 and a support 2. The thin film is further cleaned by pure water having ≥0.1MΩ/cm resistivity or the refined water as injection distilled water, and sterilized as wetted. The element 3 is hermetically sealed by a film of plastics, rubber, etc., or a formed article 5 to keep its wetted and sterilized state. As a result, a user is hardly troubled even when a fresh element is set, the element can be used after a short startup time, and the outflow of an additive, etc., is eliminated, and safety is enhanced.