The present invention is to prepare a dope (36 ) containing a TAC and a plasticizer . Impurities in the dope 36 is filtered by a filtering device (57) . In the filtration device 57 , the dope 36 flows into the flow path (58b) in the outer channel (58a) through the filter (59) . Filter (59) has a first layer formed of a sintered metal fiber (59a), a second layer (59b) and the third layer (59c). The average diameter of the metal fibers of the first layer (59a) is 8mm, the average diameter of the metal fibers of the second layer (59b) is 4mm , and the average diameter of the metal fibers of the third layer ( bottom layer ) (59c) is 20mm . Reuse by roasting for 2 hours at 400 the filter (59) . Fragments from the sintered metal fibers in the first and second layers (59a, 59b) is filtered in the third layer (59c).
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