Abstract: A primary dope containing a polymer, a solvent and an additive is fed to a filtration device in a filtrating unit. The filtration device includes a filter on which a deposit layer of a filtration aid is formed, and impurities in the primary dope are adsorbed to the deposit layer. After the filtration, a residue containing the impurities and the filtration aid on the filter, and the filtration device is filled a cleaning liquid. Then a stirrer in the filtration device is stirred so as to disperse the residue to the cleaning liquid. Thus a slurry liquid is obtained and fed to a separation device. In the separation device, the slurry liquid is fed into a strainer to separate the slurry liquid to the residue and the liquid material. The residue is dried in a drying device and the solvent vapor is recovered.