A method for producing a sulfide-based solid electrolyte which comprises: a solution making step for preparing a homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus and elemental sulfur in an organic solvent; a drying step for removing the organic solvent from the homogeneous solution to obtain a precursor; and a heat treatment step for heat-treating the precursor to obtain a sulfide-based solid electrolyte, wherein the solution making step comprises: a solution making step 1 for mixing Li 2 S and P 2 S 5 with each other in the organic solvent to prepare an Li-P-S homogeneous solution; a solution making step 2 for preparing an Li-Sn-S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur in the organic solvent, wherein the solution making step 2 comprises mixing Li 2 S, SnS and S with each other in the organic solvent to prepare the Li-Sn-S homogeneous solution, and mixing the Li-P-S homogeneous solution and the Li-Sn-S homogeneous solution with each other to prepare the homogeneous solution, wherein the sulfide-based solid electrolyte contains an LGPS-based solid electrolyte and has peaks at at least 20 = 19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å). a solution making step for preparing a homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn), elemental phosphorus and elemental sulfur in an organic solvent; a drying step for removing the organic solvent from the homogeneous solution to obtain a precursor; and a heat treatment step for heat-treating the precursor to obtain a sulfide-based solid electrolyte, wherein the solution making step comprises: a solution making step 1 for mixing Li 2 S and P 2 S 5 with each other in the organic solvent to prepare an Li-P-S homogeneous solution; a solution making step 2 for preparing an Li-Sn-S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur in the organic solvent, wherein the solution making step 2 comprises mixing Li 2 S, SnS and S with each other in the organic solvent to prepare the Li-Sn-S homogeneous solution, and mixing the Li-P-S homogeneous solution and the Li-Sn-S homogeneous solution with each other to prepare the homogeneous solution, wherein the sulfide-based solid electrolyte contains an LGPS-based solid electrolyte and has peaks at at least 20 = 19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å). a solution making step 1 for mixing Li 2 S and P 2 S 5 with each other in the organic solvent to prepare an Li-P-S homogeneous solution; a solution making step 2 for preparing an Li-Sn-S homogeneous solution that includes at least elemental lithium (Li), elemental tin (Sn) and elemental sulfur in the organic solvent, wherein the solution making step 2 comprises mixing Li 2 S, SnS and S with each other in the organic solvent to prepare the Li-Sn-S homogeneous solution, and mixing the Li-P-S homogeneous solution and the Li-Sn-S homogeneous solution with each other to prepare the homogeneous solution, wherein the sulfide-based solid electrolyte contains an LGPS-based solid electrolyte and has peaks at at least 20 = 19.90°±0.50°, 20.20°±0.50°, 26.70°±0.50° and 29.20°±0.50° in X-ray diffraction (CuKα: λ=1.5405 Å).