The present invention provides an LGPS-based solid electrolyte production method characterized by having a step in which a mixture of Li3PS4 crystals having a peak at 420±10 cm−1 in a Raman measurement and Li4MS4 crystals (M being selected from the group consisting of Ge, Si, and Sn) is heat treated at 300-700° C. In addition, the present invention can provide an LGPS-based solid electrolyte production method characterized by having: a step in which Li3PS4 crystals having a peak at 420±10 cm−1 in a Raman measurement, Li2S crystals, and sulfide crystals indicated by MS2 (M being selected from the group consisting of Ge, Si, and Sn) are mixed while still having crystals present and a precursor is synthesized; and a step in which the precursor is heat treated at 300-700° C.