To overcome the above drawbacks, we focused on a pore-filling method. Pore-filling membrane is prepared by filling a porous substrate with another material. In the case of PEMs fabricated by pore-filling, if the electrolyte is well filled into the pore space, its structure spontaneously becomes phase-separated due to the substrate structure, which has mechanical strength. In other words, a membrane with continuous proton pathways and high durability can be obtained . Furthermore, by reducing the pore size, fuel permeation can be easily reduced, which is one of the advantages . However, it has not been paid much attention as a fabrication method of organic-inorganic composite membranes.