A method of manufacturing a sulfide-based solid electrolyte through a wet process, the method comprising: preparing a slurry by adding a solvent to a mixture including lithium sulfide and a sulfide of a group 14 or group 15 element, wherein the mixture includes 60 mol% to 80 mol% of the lithium sulfide, 10 mol% to 32 mol% of the sulfide of the group 14 or group 15 element, and 4 mol% to 20 mol% of nickel sulfide, and wherein the solvent is preferably added to the mixture such that a solid content of the slurry is 10 wt% to 15 wt%; amorphizing the mixture by milling the slurry, wherein the amorphizing is preferably performed by milling the slurry under conditions of 300 RPM to 800 RPM and 4 hrs to 40 hrs by using planetary mill; drying the slurry in order to remove the solvent, preferably such that the residue of the solvent becomes more than o wt% and equal to or less than 5 wt%; and crystallizing the dried mixture by heat-treating to form the sulfide-based solid electrolyte, wherein preparing the slurry further comprising mixing nickel sulfide to the mixture, which preferably is at least one of Ni 3 S 2 , NiS and NiS 2 , and wherein the drying of the slurry comprises: primarily drying the slurry in a vacuum state for 10 mins to 20 hrs at 25°C to 60°C; and secondary drying the slurry under the following conditions: a temperature higher than a melting point of the solvent; a temperature lower than a crystallization temperature of the amorphized mixture; preferably under a condition of 130°C to 190°C; and 10 mins to 4 hrs. preparing a slurry by adding a solvent to a mixture including lithium sulfide and a sulfide of a group 14 or group 15 element, wherein the mixture includes 60 mol% to 80 mol% of the lithium sulfide, 10 mol% to 32 mol% of the sulfide of the group 14 or group 15 element, and 4 mol% to 20 mol% of nickel sulfide, and wherein the solvent is preferably added to the mixture such that a solid content of the slurry is 10 wt% to 15 wt%; amorphizing the mixture by milling the slurry, wherein the amorphizing is preferably performed by milling the slurry under conditions of 300 RPM to 800 RPM and 4 hrs to 40 hrs by using planetary mill; drying the slurry in order to remove the solvent, preferably such that the residue of the solvent becomes more than o wt% and equal to or less than 5 wt%; and crystallizing the dried mixture by heat-treating to form the sulfide-based solid electrolyte, wherein preparing the slurry further comprising mixing nickel sulfide to the mixture, which preferably is at least one of Ni 3 S 2 , NiS and NiS 2 , and wherein the drying of the slurry comprises: primarily drying the slurry in a vacuum state for 10 mins to 20 hrs at 25°C to 60°C; and secondary drying the slurry under the following conditions: a temperature higher than a melting point of the solvent; a temperature lower than a crystallization temperature of the amorphized mixture; preferably under a condition of 130°C to 190°C; and 10 mins to 4 hrs. primarily drying the slurry in a vacuum state for 10 mins to 20 hrs at 25°C to 60°C; and secondary drying the slurry under the following conditions: a temperature higher than a melting point of the solvent; a temperature lower than a crystallization temperature of the amorphized mixture; preferably under a condition of 130°C to 190°C; and 10 mins to 4 hrs. a temperature higher than a melting point of the solvent; a temperature lower than a crystallization temperature of the amorphized mixture; preferably under a condition of 130°C to 190°C; and 10 mins to 4 hrs. <EOS>