  An effective meson Lagrangian including a scalar glueball is constructed on
the base of $U(3)\times U(3)$ chiral symmetry. The glueball is introduced into
the meson Lagrangian by using the principle of scale invariance of an effective
Lagrangian and the dilaton model. The singlet-octet mixing of scalar meson
states is described by means of 't Hooft interaction. The contribution of the
scalar and pseudoscalar anomalies into the breaking of scale invariance is
taken into account. The mixing of quarkonia with the glueball is described. The
mass spectrum of scalar mesons together with the glueball and also their strong
decay widths are calculated. From comparing the obtained results with
experimental data, it follows that $f_0(1500)$ is rather a glueball, whereas
$f_0(1710)$ is a quarkonium. This accords with the results obtained in our
previous works where radially-excited scalar meson states were described. It is
shown that $\rho$-mesons play an important role in the description of glueball
decays.
