The internal structure of the thin accretion disk in the vicinity of a black hole inside the last stable orbit (r<3rg) 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, so at any way up to the sonic surface the flow is not radial. As a result in the (Euler) vertical balance equation the dynamical forces $\rho [(v\nabla) v]_\theta$ become important, these on the sonic surface being of the same order as the pressure gradient $\nabla_\theta P$. Thus, one may conclude that in the supersonic region (in particular at the black hole horizon) the thickness of the disk is determined by the form of ballistic trajectories rather than by the pressure gradient.