The present disclosure provides a method for producing a composite active material with the capability of improving coating efficiency. The present disclosure achieves an object by providing a method for producing a composite active material having an oxide active material, an oxide solid electrolyte layer that coats a surface of the oxide active material, and a sulfide solid electrolyte layer that coats a surface of the oxide solid electrolyte layer, where the method comprises a coating step of forming the sulfide solid electrolyte layer by conducting a mixing treatment such that a sulfide solid electrolyte material is mixed with the oxide active material coated with the oxide solid electrolyte layer while plastically deforming the sulfide solid electrolyte material, under a pressure decompressed to less than an atmospheric pressure.