Double-walled container manufacturing method

A double container manufacturing method comprising:

• molding a double container 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 flexible inner container layer and a rigid outer 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 at least one axial position of the rigid outer container layer has outer layer slits at positions corresponding to mating surfaces of the pair of partial molds, the flexible inner container layer being exposed through the outer layer slits in the rigid outer container layer,
• wherein distances between opposed innermost edges of a region for forming the outer layer slits in each of the mating surfaces are set in the range of 90% to 50%, with respect to an outer diameter of the parison, and
• wherein, when the pair of partial molds are combined together and moved to come close to each other to assemble the split mold, respective parts of mating surfaces of the pair of partial molds each orthogonal to a cylindrical outer surface or contact surface of the parison clamp the parison in a region corresponding to an upper half of the double container, while a central space passing through the parison is maintained.