Provided are a double container assembly comprising: a double container comprised of a relatively-rigid outer container and a flexible inner container disposed inside the outer container and capacity-variable in response to an amount of a content thereof, and capable of being easily produced using a molding apparatus which is not largely different from a conventional type; and a threaded cap, and a manufacturing method for the double container assembly, wherein the double container assembly and the manufacturing method can form an external air introducing opening in the double container significantly easily and reliably at a lower processing cost than that in conventional techniques. During the course of moving a pair of partial molds to come close to each other so as to combine them together to make up a split mold, a portion of a parison corresponding to a small-outer-diameter region of a double container is clamped by at least respective parts of mating surfaces of the pair of partial molds, to thereby form a protruding tub piece. Then, the protruding tub piece is cut off to form a space between the outer and inner containers of the double container, and an opening area of the space is sufficiently expanded through an operation of threadingly engaging the cap with the double container.