To develop the function equal to that of a large-sized extruder hard to control by parallelly arranging a plurality of small-sized extruders to extrude plastic to the large caliber pipe molding passage of a large caliber pipe molding die to allow respective extrudates to meet with each other and extruding a large caliber pipe from the die while molding the same.

Plastic is extruded from small-sized extruders 1, 1 through extrusion passages 2a, 2a and the respective extrudates are allowed to meet with each other in the confluent passage 2b of a large caliber pipe molding die 2 and extruded to a large caliber pipe molding passage 2c and discharged as a large caliber pipe 3 from the molding die 2 while molded. Since the small- sized extruders 1, 1 are parallelly arranged to extrude plastic to the large caliber pipe molding die 2 by the quantity corresponding to the wall thickness of the large caliber pipe 3 as mentioned above, the large caliber pipe 3 can be simply produced. Since a large-sized extruder is not used, the bad effect thereof, that is, difficulty of the control of an extrusion condition is avoided.