Ceramic electrolytes are expected to be a fundamental solution for the safety issue of lithium-ion batteries arising from the combustible organic electrolytes. Many researches aiming at the development of highly ion-conductive solid electrolytes have given us several types of oxide solid electrolytes: they have γ-Li 3PO 4-type, NASICON-type, perovskite-type, and garnet-type structures. Some of them have been applied to thin-film batteries, and some have comparably high Li +-ionic conductivity to liquid electrolytes. Moreover, they are chemically stable so that they will fit production of batteries. However, they have not become alternatives to the organic electrolytes. This article describes characteristics of some oxide solid electrolytes and the future prospects.