  This is the first paper of our series of high resolution (1") studies of the
massive star forming region G10.6--0.4. We present the emission line
observations of the hot core type tracers (O$^{13}$CS, OCS, SO$_{2}$) with
$\sim$0$"$.5 resolution. By comparing the results to the high--resolution
NH$_{3}$ absorption line observation, we confirm for the first time the
rotationally flattened hot toroid in the central $<$0.1 pc region, which has a
rotational axis perpendicular to its geometrical major axis.
  In addition, we present the observations of NH$_{3}$, $^{13}$CS, and
CH$_{3}$CN with $\sim$1$"$ resolution, and follow the dynamics of the molecular
accretion flow from the 0.3 pc radius to the inner 0.03 pc radius. With
reference to the rotational axis of the hot toroid, we measure the rotational
velocity from the molecular emission in the region. The results are consistent
with an envelope with a rapid decrease of the specific angular momentum from
the outer to the inner region. These new results improve the current
understanding of the molecular accretion flow in an ultracompact (UC)
H\textsc{ii} region created by the embedded O-type cluster.
