To prevent a filtrate from being stained while pleats are effectively prevented from being bonded to each other by a method wherein a synthetic resin sheet is wound around an outer periphery of the pleat, fused in a ring state by pressure welding onto the outer periphery of the synthetic resin sheet with a heater to fusion weld a crest part of the pleat to the synthetic resin sheet, and the pleats are fixed to each other by leaving a space between them.

A pleated thermally fusible filter material 4 is wound on a perforated cylinder 1 by an ordinary method. Endless processing of the filter material 4 is preferably executed by a welding method. As material of the thermally fusible filter material 4, for example, polypropylene, polyethylene, polyester, nylon, etc., are mentioned. The thermally fusible synthetic resin sheet 5 is wound by one layer or a plurality of layers on the outer periphery of the pleat part 2 of the filter material 4. The synthetic resin sheet 5 has a water permeability, and preferably a filtering function. Many rink-like welded streaks 6 are formed on the outer periphery of the synthetic resin sheet 5, and a crest part of the pleat 2 is welded to the synthetic resin sheet 5 at a part of the welded streak 6 to fix the pleats 2, 2 to each other by leaving a space between them.