PURPOSE: To obtain the titled product, even large one, efficiently by a method wherein a parison extruded from an extruder is folded in U being expanded flat to insert a core member and after a die clamping, a pressure fluid is forced into the parison to facilitate the insertion of the core body.

CONSTITUTION: A roughly semi-cylindrical parison 18 made of PVC, ABS resin or the like extruded from the die 17 of an extruder is grasped being extended flat with a grabbing member 22. Then, the grabbing member 22 is moved in the direction of the arrow on the same level extruding the parison 18 and the parison 18 hung down in U to form a large space 21 between a pair of opposed parison walls. Subsequently, a wire rod, for example, metal bar is bent rectangularly to form a core frame 12. A core rod 14 having a pair of stays 13 and 13 successively provided on the core frame 12 is moved in the direction of the arrow to position the core frame 12 within said space 21. A stamping is done with a half- split dies 20 and 20 having a peripheral groove 23 matching the shape of the core frame 12 and the stay 13, a compressed air is injected into the parison positioned in the peripheral groove from a guide groove 24 to obtain a desired product.