  We compute the vacuum energy density as a function of the quark condensate in
the interacting instanton liquid model (IILM) and examine the pattern of
dynamical chiral symmetry breaking from its behavior around the origin. This
evaluation is performed by using simulation results of the IILM. We find that
chiral symmetry is broken in the U(1)_A anomaly assisted way in the IILM with
three-flavor dynamical quarks. We call such a symmetry breaking the
anomaly-driven breaking which is one of the scenarios of chiral symmetry
breaking proposed in the context of the chiral effective theories. We also find
that the instanton-quark interaction included in the IILM plays a crucial role
for the anomaly-driven breaking by comparing the full and the quenched IILM
calculations.
