A double container manufacturing method comprising: molding a double container, by using a split mold assembled by combining a pair of partial molds together, 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; and introducing 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, wherein, when the pair of partial molds are combined together and moved to come close to each other to assemble the split mold, at least respective parts of mating surfaces of the pair of partial molds clamp the parison at horizontally opposed positions across the parison, characterised in that a portion of the parison clamped by the at least respective parts of the mating surfaces of the pair of partial molds corresponds to a neck-shoulder portion of the double container, and when the pair of partial molds are combined together and moved to come close to each other to assemble the split mold, the clamped portion of the parison is partially pushed outside clamp regions of the pair of mating surfaces to form a clearance-caused burr through a pinched section while the flexible inner container layer and the rigid outer container layer are maintained in an integral state, wherein the clearance-caused burr is removed so that the rigid outer container layer is disconnected to form two outer layer slits, and the flexible inner layer is exposed through each of the outer layer slits, such that, when a volume of an inner space defined by the flexible inner container layer decreases, the flexible inner container layer is separated from the rigid outer container layer, and said external air enters into the resulting space between the rigid outer container layer and the flexible inner container layer through the outer layer slits. molding a double container, by using a split mold assembled by combining a pair of partial molds together, 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; and introducing 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, wherein, when the pair of partial molds are combined together and moved to come close to each other to assemble the split mold, at least respective parts of mating surfaces of the pair of partial molds clamp the parison at horizontally opposed positions across the parison, characterised in that a portion of the parison clamped by the at least respective parts of the mating surfaces of the pair of partial molds corresponds to a neck-shoulder portion of the double container, and when the pair of partial molds are combined together and moved to come close to each other to assemble the split mold, the clamped portion of the parison is partially pushed outside clamp regions of the pair of mating surfaces to form a clearance-caused burr through a pinched section while the flexible inner container layer and the rigid outer container layer are maintained in an integral state, wherein the clearance-caused burr is removed so that the rigid outer container layer is disconnected to form two outer layer slits, and the flexible inner layer is exposed through each of the outer layer slits, such that, when a volume of an inner space defined by the flexible inner container layer decreases, the flexible inner container layer is separated from the rigid outer container layer, and said external air enters into the resulting space between the rigid outer container layer and the flexible inner container layer through the outer layer slits.