A Hamiltonian description of the physical system composed of a dust shell interacting with the gravitational field is considered. In the spherically symmetric case, the phase space of the system is effectively reduced with respect to the Gauss--Codazzi constraints. The Hamiltonian of the system (numerically equal to the value of the A.D.M. mass) is explicitly calculated in terms of the ``true degrees of freedom'', i.e. as a function on the reduced phase space.

PACS numbers: 04.20.Fy