A method for producing an electrochemically advantageous lithium ion-conductive solid electrolyte with high ionic conductivity, low electronic conduction and electrochemical stability is disclosed. The method comprises the steps of synthesizing lithium sulfide by reacting lithium hydroxide with a gaseous sulfur source at a temperature of not less than 130.degree. C. and not more than 445.degree. C., thermally melting plural compounds containing at least silicon sulfide and the synthesized lithium sulfide, and cooling the molten mixture. The silicon sulfide is synthesized by the steps of adding a silicon powder to molten sulfur while stirring to disperse the silicon powder in the molten sulfur and heating the silicon powder-dispersed sulfur in a reaction chamber under reduced pressure.