PURPOSE: To manufacture a hollow body having a complicate shape with high efficiency as a product with a smooth surface through an injection molding method by forming a plurality of gas-body filler holes to a mold.

CONSTITUTION: A mold cavity having the same shape as a parabolic antenna 6 is filled with a molten resin in quantity smaller than or the same quantity as the content volume of the mold cavity having the same shape as the parabolic antenna 6 from an injection fillter hole 1. A gas for pressing is injected into the molten resin from a primary pressing port 2 formed to an annular thick wall section 7, and hollow sections 4 are formed successively in the annular thick wall section 7 as a primary pressing process. The pressure of the hollow sections 4 are lowered once, a gas for pressing is injected into a non- penetrating section 5 from a secondary pressing port 3 positioned at approximately the center of the non-penetrating section 5, and hollow sections are shaped to the non-penetrating section 5 as a secondary pressing process. The hollow sections successively formed in the non-penetrating section 5 from the secondary pressing port 3 reach both ends approximately simultaneously, thus easily manufacturing a molded form with a continuous hollow section.