The invention provides a process for producing DNA nanocages, characterized by comprising a step of two-dimensionally assembling three types of oligonucleotides by hybridization to form tridirectionally branched double-strand DNA having self-complementary chains in the terminals, and a step of three-dimensionally self-organizing the resulting tridirectionally branched double-strand DNAs so as to consume all of the self-complementary terminals. Since the DNA nanocages according to the invention can easily be formed from DNAs by one-step procedure and include nanoparticles in the interior, the DNA nanocages are significantly effective for the development of functional materials using DNAs.