The present invention provides: a carbon nanohorn composite including a carbon nanohorn, a substance encapsulated in the carbon nanohorn, and a polyamine adsorbed by chemical reaction firmly to a surface functional group present on the opening part on the surface of the carbon nanohorn, wherein the release amount and release rate of the encapsulated substance can be controlled using the difference in size, substituent or three-dimensional structure of the polyamine, which is used as a plug; a method of controlling the release of the encapsulated substance; and a process for producing the carbon nanohorn composite. The release amount and release rate of the substance encapsulated in the carbon nanohorn composite is controlled by selecting a polyamine molecule, which plugs the opening part formed in the carbon nanohorn by oxidation, by its size, substituent or three-dimensional structure.