Abstract: A solid electrolyte comprises a ramsdellite-type crystal structure and has low activation energy of lithium ions and good lithium ion conductivity. The solid electrolyte is represented by the general formula Li4x?2a?3b?c?2dSn4?x?c?dM(II)aM(III)bM(V)cM(VI)dO8 [wherein M(II) is a divalent cation, M(III) is a trivalent cation, M(V) is a pentavalent cation, and M(VI) is a hexavalent cation, 0?x?1.33], wherein in the general formula, 0<a+b+c+d, 0?a+b?x, 0?c+d<0.9, and 3x?a?2b?c?2d?2. The all-solid-state battery includes the solid electrolyte in at least one layer of the positive electrode layer, negative electrode layer, and solid electrolyte layer. The method of making the solid electrolyte includes a step of preparing a mixed powder as a raw material and heating with microwave irradiation.