A composite solid electrolyte including a first solid electrolyte layer including a halide solid electrolyte, a second solid electrolyte layer including an oxide solid electrolyte, a lithium ion-conductive interlayer between the first solid electrolyte layer and the second solid electrolyte layer, wherein the lithium ion-conductive interlayer includes a composition in which a reaction energy of the lithium ion-conductive interlayer with respect to the first solid electrolyte is −50 meV/atom or greater.