Metal nanoparticles having improved migration resistance are provided. The present invention relates to a method for manufacturing composite nanoparticles including obtaining composite nanoparticles containing at least silver and copper in a single particle by heat treating a mixture containing an organic silver compound and an organic copper compound at a temperature of 150° C. or more in a non-oxidative atmosphere in the presence of a tertiary amine compound represented by the general formula R1R2R3N (wherein R1 through R3 are optionally substituted alkyl groups or aryl groups that may be the same or different, R1 through R3 may be linked in a ring, and the number of carbon atoms in each of R1 through R3 is 5 through 18 and may be the same or different).