A method of producing a composite nanoparticle (M-AxOy), having: generating, in an inert gas, an alloy (A-M) nanoparticle, which contains 0.1 at. % to 30 at. % of a noble metal (M), with the balance being a base metal (A) and inevitable impurities, and which has a particle size of 1 nm to 100 nm, to heat the alloy (A-M) nanoparticle and to bring the alloy (A-M) nanoparticle into contact with a supplied oxidizing gas during transportation of the alloy (A-M) nanoparticle with the inert gas, to oxidize the base metal component (A) in the floating alloy (A-M) nanoparticle, and to phase separate into the thus-oxidized base metal component (AxOy) and the noble metal component (M), to thereby obtain a composite nanoparticle (M-AxOy) having one noble metal particle (M) combined to the surface of a particulate base metal oxide (AxOy).