A sulfide-based inorganic solid electrolyte that suppresses the reaction between silicon sulfide and metallic lithium even when the electrolyte is in contact with metallic lithium, a method of forming the electrolyte, and a lithium battery's member and lithium secondary battery both incorporating the electrolyte. The electrolyte comprises Li, P, and S without containing Si. It is desirable that the oxygen content vary gradually from the electrolyte to the lithium-containing material at the boundary zone between the two members when analyzed by using an XPS having an analyzing chamber capable of maintaining a super-high vacuum less than 1.33.times.10.sup.-9 hPa and that the oxygen-containing layer on the surface of the lithium-containing material be removed nearly completely. The electrolyte can be formed such that at least part of the forming step is performed concurrently with the step for etching the surface of the substrate by irradiating the surface with inert-gas ions.