To provide nanoparticles comprising a polymer aggregate expected as a constituent material of an electrophotographic developer, printing ink, architectural coating, etc., by making use of a photodegradation reaction, and to provide a new technique for producing nanoparticles by photodegradation from which application expansion to an optical memory material, an optical sensor, etc., is expected.

A block copolymer comprising a styrene-based polymer and a tert-butoxy group-containing polymer is irradiated with light in the presence of a photoacid generator, whereby the copolymer partly degrades and aggregates to form nanoparticles of several ten nm.