Belov in [2] gave necessary and sufficient condition for rotational surface generated by a special quadrangular meridian, to be rigid. Belov's theorem disproved the hypothesis of Boyarski that each toroid rotational surface with convex meridian is rigid. We give another proof of Belov's theorem. The field of infinitesimal bendings is determined, the rotational field is obtained too. The method, used here, can be applied in a case of every rotational surface generated by a simple polygon [6].