PURPOSE: To improve a quality of a product by unifying a thickness of a parison at the time of blow molding, by deforming beforehand the parison into a shape of a cavity by a preforming mold.

CONSTITUTION: Two sheets of sheetlike parisons 14 are extruded until the lower ends of the parisons 14 each arrives at the lower end positions of both a right and left molds 11, 12 by an extruding machine 15 under a state wherein a preforming mold 13 is placed between the parison 14, and two sheets of sheetlike fabrics 16, which are among the molds 11, 12 and the parisons 14, are drawn out by a supply device 7 until the lower ends of them arrive at the lower ends of both the right and left molds. Then both the right and left molds 11, 12 are moved to a preforming mold 13 side and the parisons 14 and fabrics 16 are held among the molds 11, 12 and preforming mold 13 while they are being deformed into almost shaped of cavities 11a, 12a beforehand. After completion of the holding, compressed air is blown off further through each of blow-off openings 13c of the preforming mold 13 and parisons 14 and fabrics 16 are pressed by both the molds, through which they are molded into the shapes running along the cavities 11a, 12a.