A filter is formed by a sintered non-woven metal fiber cloth adapted to be installed in a liquid passage, and, in the filter, a substance sticking to the surface of metal fibers which is present in the outermost part of the filter and in the vicinity thereof is removed together with the surface layer of the metal fibers. The liquid medium comprises at least one species of phototrophic or mixotrophic organism therein. As a result, the pressure plate 65 moves back against energizing force of the cone disk spring, and the initial torque is cancelled.