Abstract: Provided is a sulfide-based solid electrolyte with high lithium ion conductivity. The sulfide-based solid electrolyte may be a sulfide-based solid electrolyte, wherein the sulfide-based solid electrolyte comprises a lithium (Li) element, a phosphorus (P) element, a sulfur (S) element and a halogen element, and it has a LGPS-type crystal structure, and wherein a ratio (P4d/P2b) between a proportion (P4d) of an area of a peak assigned to phosphorus atoms occupying 4d sites in the crystal structure and a proportion (P2b) of an area of a peak assigned to phosphorus atoms occupying 2b sites in the crystal structure, both of which are peaks observed in a 31P-MAS-NMR spectrum of the sulfide-based solid electrolyte, is 1.77 or more and 2.14 or less.