A dope (24) containing a solid electrolyte and an organic solvent is cast onto a web (111) so as to form a casting membrane (24a) thereon. A first blower unit (91) applies dry air to the casting membrane (24a). After a lapse of 60 seconds from the casting, a skin layer (24c) is formed at an exposed surface (24b) side of the casting membrane (24a). The casting membrane (24a) with the web (111) is brought into contact with a first liquid (65a), and thereby reducing a remaining solvent amount of the casting membrane (24a). The casting membrane (24a) is then peeled from the web (111) as a membrane (62). In a tenter drier (64), the membrane (62) is dried while it is stretched. The membrane (62) is then brought into contact with a second liquid (66a). In a drying chamber (69), the membrane (62) is further dried.