To obtain a three-dimensional structure serving as an artificial seaweed bed or an artificial fish-breeding reef which has a simple structure, an improved handleability and excellent organism growth function as a seaweed bed function when especially applied to a small-scale river, pond, lake and marsh and ocean.

(1) The three-dimensional structure comprises a synthetic fiber containing at least partially a modified thermoplastic polyvinyl alcohol and has 0.4-0.98 void index K and 2-50 mm thickness in a woven or knitted fabric obtained by connecting both sides of a ground weave with a connecting yarn. (2) The three-dimensional structure comprises a thermoplastic filamentous material containing at least part of a modified thermoplastic polyvinyl alcohol and has 0.4-0.98 void index K and 2-50 mm thickness in a three-dimensional structure obtained by irregularly entangling a fibrous material. The artificial seaweed bed and the artificial fish-breeding reef are composed of the three-dimensional structure described in (1) or (2).