A contrast agent characterized in that each of carbon nanohorns forming a carbon nanohorn aggregate has an opening at the side wall or tip, wherein a metal M (at least one metal selected from among paramagnetic metals, ferromagnetic metals, and superparamagnetic metals) or a compound of the metal M is incorporated in or dispersed on each of the carbon nanohorns. A contrast agent characterized in that it contains a Gd oxide. There is provided a contrast agent, which can be mass-produced easily, and satisfies the requirement of low toxicity and enables microscopic diagnoses when used for MRI. A contrast agent characterized in that is contains a carbon nanohorn aggregate.
