Two-dimensional unsteady natural convection in enclosures of arbitrary cross section is considered. The convection equations in the Boussinesq approximation are written in a curvilinear nonorthogonal coordinate system, in which the boundaries of the region investigated coincide with the coordinate lines. The problem is solved numerically in the physical variables on the basis of a multistep, completely implicit finite-difference method with decoupling of the physical processes and space variables. The numerical modeling of the unsteady convection process in a cylindrical enclosure, whose cross section corresponds to part of a circle enclosed between two equal and parallel chords, is examined by way of example.