  Mapping the central 5 pc region of the nearby radio galaxy NGC 6251 with a
0.2 pc resolution using VLBI at two radio frequencies, 5 GHz and 15 GHz, we
have found the sub-pc-scale counter jet for the first time in this radio
galaxy. This discovery allows us to investigate the jet acceleration based on
the relativistic beaming model (Ghisellini et al. 1993).
