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

The Decrease of Specific Angular Momentum and the Hot Toroid Formation: The Massive Clump G10.6-0.4