This invention provides a finely particulate composite wherein a fine particle of block copolymer having a poly(ethyleneglycol) chain segment and a poly(meth)acrylate ester chain segment containing, on its side chain, a tertiary amino group and/or a secondary amino group has, encapsulated therein, a carbon compound, e.g., fullerene or carbon nanotube which may contains, encapsulated therein, a metal. This composite is water-soluble, and is usable as an active oxygen scavenger. This composite, when it has a metal encapsulated therein, is usable as a contrast medium.