Tubular parison of a molten resin to such an extent as to melt undergoes a pre-blow with a blow molding apparatus, whereupon the blown tubular parison is clamped between dies of a blow molding apparatus, and also, is mutually weld together with parts of the inside faces of the tubular parison to form a multiple of isolated hollows in the parison. Gas is fed through a multiple of gas feed nozzles which are provided in the die into the hollow portions. A number of the hollow portions are opened in a second process to form a functional part.