A method for manufacturing a blow-molded double container assembly comprising a double container (10, 610, 710) and a threaded cap, wherein the double container is molded, using a split mold as a combination of a pair of partial molds, from a parison comprising at least two layers of an inner layer and an outer layer forming, respectively, a rigid outer container layer and a flexible inner container layer which are separable from each other after molding, the double container (10, 610, 710) being configured to introduce external air into a space between the rigid outer container layer and the flexible inner container layer to enable a capacity of the flexible inner container layer to be reduced, while retaining a shape of the rigid outer container layer, the method comprising: a step A of: forming a large-outer-diameter region having a given outer diameter, in a mouth portion (12, 612, 712) or a container body portion (18, 618, 718) of the double container (10, 610, 710) or a portion of the double container (10, 610, 710) therebetween, and a small-outer-diameter region having an outer diameter less than that of the large-outer-diameter region, in a portion of the double container (10, 610, 710) between the large-outer-diameter region and a cap-receiving threaded portion (96, 696, 796) of the double container; and, during a course of moving the pair of partial molds to come close to each other so as to combine them together to make up the split mold, clamping a portion of the parison corresponding to the small-outer-diameter region by at least respective parts of mating surfaces of the pair of partial molds, to thereby form two clearance-caused burrs; a step B of removing the clearance-caused burrs formed in the step A such that the outer layer of the parison is disconnected to form two slits in the outer layer and the inner layer of the parison is exposed through the slit; and a step C of forming the threaded cap such that, when the threaded cap is threadingly engaged with the cap-receiving threaded portion (96, 696, 796), a lower end of the threaded cap is brought into contact with the large-outer-diameter region of the double container (10, 610, 710) and applies a tension to the small-outer-diameter region from which the clearance-caused burrs have been removed, in a direction causing the cap-receiving threaded portion (96, 696, 796) and the large-outer-diameter region to be pulled away from each other thereby opening the slits to enable external air to flow into a space between the inner layer and outer layer through the slits. a step A of: forming a large-outer-diameter region having a given outer diameter, in a mouth portion (12, 612, 712) or a container body portion (18, 618, 718) of the double container (10, 610, 710) or a portion of the double container (10, 610, 710) therebetween, and a small-outer-diameter region having an outer diameter less than that of the large-outer-diameter region, in a portion of the double container (10, 610, 710) between the large-outer-diameter region and a cap-receiving threaded portion (96, 696, 796) of the double container; and, during a course of moving the pair of partial molds to come close to each other so as to combine them together to make up the split mold, clamping a portion of the parison corresponding to the small-outer-diameter region by at least respective parts of mating surfaces of the pair of partial molds, to thereby form two clearance-caused burrs; a step B of removing the clearance-caused burrs formed in the step A such that the outer layer of the parison is disconnected to form two slits in the outer layer and the inner layer of the parison is exposed through the slit; and a step C of forming the threaded cap such that, when the threaded cap is threadingly engaged with the cap-receiving threaded portion (96, 696, 796), a lower end of the threaded cap is brought into contact with the large-outer-diameter region of the double container (10, 610, 710) and applies a tension to the small-outer-diameter region from which the clearance-caused burrs have been removed, in a direction causing the cap-receiving threaded portion (96, 696, 796) and the large-outer-diameter region to be pulled away from each other thereby opening the slits to enable external air to flow into a space between the inner layer and outer layer through the slits.