  We give a construction of a nuclear $C^\ast$-algebra associated with an
amalgamated free product of groups, generalizing Spielberg's construction of a
certain Cuntz-Krieger algebra associated with a finitely generated free product
of cyclic groups. Our nuclear $C^\ast$-algebras can be identified with certain
Cuntz-Krieger-Pimsner algebras. We will also show that our algebras can be
obtained by the crossed product construction of the canonical actions on the
hyperbolic boundaries, which proves a special case of Adams' result about
amenability of the boundary action for hyperbolic groups. We will also give an
explicit formula of the $K$-groups of our algebras. Finally we will investigate
the relationship between the KMS states of the generalized gauge actions on our
$C^\ast$ algebras and random walks on the groups.
