PURPOSE: To facilitate an assembly after molding and manufacture the large amount of a sewage disposal tank of high strength at a low cost by shaping an opening part for inserting a partitioning plate at one end of a tube in time of blow molding.

CONSTITUTION: A vinylchloride resin pipe 11 is blow molded between the upper and lower metal molds 12, 13. The clamps 14, 14, 14', 14' are attached to both right and left ends of the upper and lower metal molds 12, 13 and an inner metal mold 15 capable to advance and retreat in the axial direction of the vinyl chloride resin pipe 11 to be molded is provided at its one opening end. Both ends of the vinyl chloride resin pipe 11 are bent inside by the clamps, while an opening part 19 for an insertion is shaped at one end a press-bonded part 20 is shaped at another end. Blow molding is performed by sending a compressed air into the vinyl chloride resin pipe 11 from a compressed air introducting pipe 21 attached to the inner metal mold 15.