I. K. Sabitov [6] made conjecture, that for any infinitesimal $C^1$ smooth bending of a closed surface the volume, bounded by it, will be stationary, i.e. that the variation of this volume is zero. V. A. Aleksandrov has proved in [1] that the mentioned conjecture is true for the rotational surface of the type 0 or 1 with $C^1$-smooth meridian not, containing a segment perpendicular to the axis of rotation. At the end of the work [1] the following question is formulated: Is the volume, bounded by a piecewise-smooth surface of rotation, always stationary under its infinitesimal bendings? I11 the present work we prove that the answer to this question is positive. Besides, by a direct calculation of the volume variation, we confirm this in the case of Belov's rotational toroid [2].