Studies on phase transition of a model ionic conductor thin film sandwiched with insulators are theoretically carried out in terms of the interacting lattice gas model, and become a possibility if a low-temperature confinement of superionic conductors is realized. Due to a number of defects existing in the interface region, the defect-induced phase transition is promoted. giving rise to the order-disorder phase transition front. This suggests that a sandwiched superionic conductor thin film or superionic fine particles can be realized well below the bull; superionic transition temperature.