First, an ionic liquid is placed on a glass slide, which is then installed in an evaporation apparatus, and a metal (for example, indium) is mounted as a target material at a position facing the ionic liquid, followed by sputter deposition of the metal. After sputtering, the ionic liquid containing nanoparticles dispersed therein is recovered. The nanoparticles are solid nanoparticles. Next, the ionic liquid containing the solid nanoparticles dispersed therein is placed in a test tube and then oxidized by heating in air at 250° C. for 1 hour. As a result, hollow nanoparticles having cavities formed in core portions of the solid nanoparticles are produced.