Among the next-generation battery technologies, all-solid-state lithium batteries (ASLBs) are the most attractive because of the high safety and high energy density. The critical difference between ASLBs and conventional lithium-ion batteries (LIBs) is the replacement of the liquid electrolyte with a solid electrolyte (SE). Thus, for battery development, the investigation of ionic conductivities of SEs is essential. Sulfide-type ion conductors are representative SEs having high ionic conductivities and are ductile. However, sulfide-type SEs suffers from H2S gas release and degradation when exposed to the moisture in the air, and, as a result, the study and optimization of the fabrication parameters is challenging. In this study, we fabricated a polymer-in-ceramic SE as a thin, large-area, free-standing SE. Crucially, to optimize the fabrication conditions, we used a model inorganic particles that do not suffer from the moisture sensitivity typical of sulfide-based SEs. Interestingly, the ionic conductivity of the polymer-in-ceramic SE changed with applied pressure, behavior unlike that of a conventional pellet-type SEs prepared from sulfide powders. To understand this phenomenon, we carried out digital twinned 3D structure simulation analysis, which revealed changes in the specific contact area and distribution of ionic density in the polymer-in-ceramic SE. As a result, we propose a model composition that will facilitate the exploration of polymer-in-ceramic SEs and their characteristics and, thus, enhance the practical use of ASLBs.