  The internal structure of the thin transonic disk accreting onto a
nonrotating black hole inside the last stable orbit ($r < 3r_{\rm g}$) is
considered within the hydrodynamical version of the Grad-Shafranov equation. It
is shown that in the vicinity of the sonic surface takes place a sharp
diminishing of the disk thickness. As a result, in the vertical balance
equation the dynamical forces $\rho[({\bf v}\nabla){\bf v}]_{\theta}$ become
important, these on the sonic surface being of the same order as the pressure
gradient $\nabla_{\theta} P$. In the supersonic region the thickness of the
disk is determined by the form of ballistic trajectories rather than by the
pressure gradient.
