Disclosed is a lithium ion-conductive sulfide-based solid electrolyte which includes nickel sulfide and, accordingly, the solid electrolyte can obtain a novel structure and performance.More particularly, the sulfide-based solid electrolyte includes lithium sulfide (Li2S), diphosphorus pentasulfide (P2S5), and nickel sulfide (Ni3S2) in a specific ratio by mol % and exhibits a novel crystal structure due to nickel (Ni). Accordingly, the sulfide-based solid electrolyte has greater lithium ion conductivity than an conventional sulfide-based solid electrolyte and a stable crystal structure.