To increase the degree of reproduction of filters of filter devices by washing the filters sufficiently without applying an excessive counter pressure and to prevent breakage of the filters.

Dope for solution film making is filtered by one of the filter devices F4 and F5 which are mutually connected in parallel and have metal filters, and the other which is used is backflow-washed by a solvent. The filtered dope is sent to a film making process to be subjected to continuous film making. The flow rate of the washing solvent to the filter device F4 in a first liquid supplying passage L1 is adjusted so that the regulating valve V51 of a second liquid supplying passage L2 is fully opened at the start to control counter pressure applied thereafter to the filter not to exceed a counter pressure limit. The flow rate of the solvent is made at least 10 times of a filtration flow rate, and the total volume of the solvent is made at least 50 times of the volume of the dope left in the filter device F4. The residual dope is returned to a third stock tank T3. In this way, filter is washed while continuous film making is done, and the degree of reproduction can be improved without breaking a filter medium.