To utilize the mechanism in the region of a filter mounted in a fluid discharge passage.

A filter cartridge mounted in a fluid discharge conduit and capable of being drawn out of the conduit through an opening 8 in the wall of the conduit 1 is replaced by this method. Namely, the cartridge 6 is fitted into a bag 14 which is opened only at the level of the opening 8 at the time of the drawing out.