In a casting membrane forming process (63), a dope containing a solid electrolyte is cast on a casting belt (93) from a casting die. On the casting belt, a casting membrane (61) is formed. In a casting membrane drying process (66a), the casting membrane (61) is dried. In a solvent contacting process (66b), the casting membrane (61) contacts a poor solvent of the solid electrolyte. By a self-supporting property developing process (66), a self-supporting property is given to the casting membrane (61) for a short time. The casting membrane (61) is peeled from the casting belt (93) as a precursor membrane (67). The precursor membrane (67) is immersed into water. After the precursor membrane (67) is dried, a proton substitution is performed to obtain a hydrogen-substituted membrane (75). By washing and drying of the hydrogen-substituted membrane (75), a solid electrolyte membrane (79) is obtained.