PURPOSE: To obtain a gel made of a specific core-sheath fiber gel containing a specific component, formed in the form of a net or a reed screen free from pollution of water area and having highly durable waste-water cleaning function,

CONSTITUTION: An aqueous solution containing a water-soluble polysaccharide (preferably alginic acid salt, etc.) gelatinizable by the contact with a water-soluble synthetic polymer (e.g. PVA) and a cation (preferably calcium chloride solution, etc.) is extruded through a nozzle together with a fiber (preferably synthetic fiber, etc.) and brought into contact with an aqueous solution containing the cation to form a gel of the water-soluble polymer as the sheath part of a core-sheath fibrous gel containing the fiber as the core. The gel is processed and formed in the form of a net or a reed screen. Since the net can clean a water area without damaging the fish body, it is suitable for a fish culture net or a stationary trap net. The gel having the form of reed screen keeps the cleaning function in high durability because the gelatinous material does not fall off from the screen over a long period.