Abstract: Provided is a method of producing a composite solid electrolyte. The method includes step S10 of producing an oxide-based solid electrolyte membrane by electrospinning a mixture including an oxide-based solid electrolyte precursor and a polymer, step S20 of producing an oxide-based solid electrolyte support by removing the polymer inside the oxide-based solid electrolyte membrane, and step S30 of causing the oxide-based solid electrolyte support to be impregnated with a sulfide-based solid electrolyte using a sulfide-based solid electrolyte precursor solution including a sulfide-based solid electrolyte precursor and a solvent.