  Supermassive black hole binaries may exist in the centers of active galactic
nuclei like quasars and radio galaxies and mergers between galaxies may result
in the formation of supermassive binaries during the course of galactic
evolution. Using the very-long-baseline interferometer, we imaged the radio
galaxy 3C 66B at radio frequencies and found that the unresolved radio core of
3C 66B shows well-defined elliptical motions with a period of 1.05 +/- 0.03
years, which provides a direct detection of a supermassive black hole binary.
