A substance-encapsulating carbon nanohorn aggregate which has improved chemical stability by isolating the encapsulated substance from outside and which is useful as a targeting material which can be led from the outside of the body or as a contrast medium by holding the encapsulated substance in an aggregated form, and a process for producing the same are provided. The substance-encapsulating carbon nanohorn aggregate is characterized in that the encapsulated substance is aggregated in a central part of the carbon nanohorn aggregate or a neighborhood thereof with being isolated from outside. The process includes aggregating a substance to be encapsulated in a central part or a neighborhood thereof by a heat treatment.