In a manufacturing method for a delamination method, in a second injection molding including forming a second layer on an inner peripheral side of a first layer of a preform having a bottomed cylindrical shape, a second resin material is guided from an opening portion formed in the first layer toward the inner peripheral side of the first layer, and a locking portion protruding from the opening portion toward an outer peripheral side of the first layer is integrally formed with the second layer.