  In the present paper we propose a mechanism of the structural instability
with a periodic charge ordering in two-dimensional isotropic conductors with a
closed Fermi surface which completely excludes the conventional nesting
mechanism. We show that the structural instability in such conductors may arise
as a topological reconstruction under which the initially closed Fermi surface
is transformed into an open one. We have found that the order parameter of the
charge ordering ground state may exceed one hundredth of the Fermi energy.
Furthermore, this charge ordering is a quantum phase transition with respect to
the dimensionless coupling constant $\lambda$ related to the mechanism that
drives the band reconstruction (e. g. electron-phonon coupling), with the
critical value given by $\lambda_c= (1+2/\pi)^{-1}$. Preliminary estimations
show that the suggested mechanism can be the origin of density waves observed
in such materials as high$-T_c$ cuprates or graphite intercalates.
