PURPOSE: To manufacture a strong pressure resistant solid void structure having a cylindrical shape, a tabular shape, a box shape, etc., continuously by extruding a thermoplastic raw material in a filamentous manner and mutually welding the extruded raw material while forming solid spaces having irregular surfaces.

CONSTITUTION: A mold 2, in which the terminal sections of nozzles 1 are bored in a sectional shape using a round shape as a basis, is mounted on a die, and a thermoplastic raw material is extruded and cooled and formed in a filament diameter of approximately 3-10mm. Filaments are curved in cooling water and welded mutually with adjacent other filaments at that time, the strength of the filaments is reinforced and a solid net structure is formed, and the sectional shape having a cylindrical shape, a thin plate shape, a box shape, etc., is formed. Accordingly, projected wave power can be damped by waveform variation by the surfaces of the filaments of the solid net structure and flow-rate phase difference by the void sections of the solid structure as a wave dissipating material.