  We present six-year multi-wavelength monitoring result for broad-line radio
galaxy 3C 120. The source was sporadically detected by Fermi-LAT and after the
MeV/GeV gamma-ray detection the 43 GHz radio core brightened and a knot ejected
from an unresolved core, implying that the radio-gamma phenomena are physically
connected. We show that the gamma-ray emission region is located at sub-pc
distance from the central black hole, and MeV/GeV gamma-ray emission mechanism
is inverse-Compton scattering of synchrotron photons. We also discuss future
perspective revealed by next-generation X-ray satellite Astro-H.
