A composite electrolyte membrane according to the present invention has a phase-change layer disposed on a surface facing an electrode such as a positive electrode. The phase-change layer can lower the resistance at the interface of the electrolyte membrane and the electrode by liquefying a filling material through heating by raising the battery internal temperature, for example, and filling a separated portion (dead space), that is, a portion in which physical contact is severed between the positive electrode and the composite electrolyte membrane.