Lithium ion conductive composite solid electrolyte and solid-state battery using the same

The present invention provides a lithium ion conductive composite solid electrolyte including an oxide-based crystalline solid electrolyte and a sulfide-based solid electrolyte, wherein the oxide-based solid electrolyte is a garnet-type solid electrolyte represented by a general formula of LiAlLaZrMO, wherein M is at least one element selected from the group consisting of niobium (Nb), tantalum (Ta), and tungsten (W), y satisfies 0≤y≤1, and z satisfies 0≤z