  The variational principle for a spherical configuration consisting of a thin
spherical dust shell in gravitational field is constructed. The principle is
consistent with the boundary-value problem of the corresponding Euler-Lagrange
equations, and leads to ``natural boundary conditions''. These conditions and
the field equations following from the variational principle are used for
performing of the reduction of this system. The equations of motion for the
shell follow from the obtained reduced action. The transformation of the
variational formula for the reduced action leads to two natural variants of the
effective action. One of them describes the shell from a stationary interior
observer's point of view, another from the exterior one. The conditions of
isometry of the exterior and interior faces of the shell lead to the momentum
and Hamiltonian constraints.
