As the application area of batteries reaches much larger scales that equip a large number of cells, the safety concerns become more important due to the increased potential risk of explosions. The use of non-flammable inorganic solid electrolytes brings about the paradigm shift in battery technology and emerging all solid-state batteries as a new class of highly safe batteries. We are focusing on the synthesis and processing of solid-state oxide electrolytes and electrode-electrolyte interface engineering to realize highly safe and stable solid-state batteries.