The present invention relates to an average-density-adjustable structure and more specifically provides a structure the average density of which is adjusted by changing the material of the structure and the size of a void formed therein and which can thereby float on the surface of or in a liquid and can easily bond with or change a material present in a gas or liquid by being equipped with a first material, which is one among an organic catalyst, an inorganic catalyst, a microorganism, and a biomolecule.