A polymer solution (21) is prepared by dissolving poly-.ε.-caprolactone and amphipathic polyacrylamide in an organic solvent. The polymer solution (21) is cast from a casting die (25) onto a casting belt (26) to form a casting film (40). Air (35) from a blowing and suctioning device (34) is sent to the casting film (40) to generate droplets (44). The relative speed of drying air (37) to the moving speed of the casting belt (26) is adjusted at 5 m/min in parallel flow. The water in the drying air (37) is condensed in the casting film (40) to form the droplets (44). The droplets (44) are evaporated after evaporating the organic solvent in the casting film (40), so that a honeycomb-structure film (12) can be obtained.