Let $G$ be a discrete subgroup of $PU(1,n; \mathbf{C})$. For
a boundary point $y$ of the Siegel domain, we define the generalized
isometric sphere $I_y(f)$ of an element $f$ of $PU(1,n; \mathbf{C})$.
By using the generalized isometric spheres of elements of $G$,
we construct a fundamental domain $P_y(G)$ for $G$, which
is regarded as a generalization of the Ford domain. And we
show that the Dirichlet polyhedron $D(w)$ for $G$ with center
$w$ convereges to $P_y(G)$ as $w \rightarrow y$. Some results
are also found in [5], but our method is elementary.