Kubo, T. JACoW Geneva, Switzerland Structures of alternating layers of superconducting (SC) and insulating layers formed on a bulk SC are actively discussed these years, because of their great possibility in realizing a high-field and high-Q SCRF cavity. In 2013, theoretical studies of an ideal system with a single SC layer and a single insulator layer formed on a bulk SC were carried out, by which appropriate combinations of materials and optimum thicknesses of SC and insulator layers that can enhance field limit were revealed. A more realistic model with nano-scale surface defects on the SC layer was also studied in 2014. As of this year, a sample-experiment-oriented theoretical study is in progress towards proof-of-concept experiments. Theories are becoming realistic every year. In this talk, I will review progresses after 2013 and provide theoretical equipages to tackle experiments of the multilayer coating(*).