PURPOSE: To continuously obtain a high expansibility foam with an odd-shaped section by a method wherein an inner mold with an odd-shaped section is arranged in a molding die and expansion is carried out while keeping the pressure in the interior of a molded body below the atmospheric pressure.

CONSTITUTION: Expansible thermoplastic resin composition 1 in melted and sent in a molding die 5 directly connected to a heat melt zone 3 and finally discharged from a molding nozzle 9, in the center of which an inner mold 7 is separatedly arranged by bridges 8 provided in the center of a cylindrical outer mold 6. The composition 1 is expanded under the condition that the interior of a molded body 12 just after the extrusion molding is evacuated through a suction pipe 10, one end of which is open near the molding nozzle 9 by means of an evacuator 11 so as to nearly eliminate the hollow portion 13 in the section of the molded body in order to continuously obtain a tubular foam with desired odd- shaped section.