  The structure of the M87 jet, from milli-arcsec to arcsecond scales, is
extensively investigated, utilizing the images taken with the EVN, MERLIN and
VLBA. We discover that the jet maintains a parabolic streamline over a range in
sizescale equal to $10^{5}$ times the Schwarzschild radius. The jet then
transitions into a conical shape further downstream. This suggests that the
magnetohydrodynamic jet is initially subjected to the confinement by the
external gas which is dominated by the gravitational influence of the
supermassive black hole. Afterwards the jet is then freely expanding with a
conical shape. This geometrical transition indicates that the origin of the
HST-1 complex may be a consequence of the over-collimation of the jet. Our
result suggests that when even higher angular resolution is provided by a
future submm VLBI experiment, we will be able to explore the origin of active
galactic nuclei jets.
