  The partial restoration of chiral symmetry in nuclear medium is investigated
in a model independent way by exploiting operator relations in QCD. An exact
sum rule is derived for the quark condensate valid for all density. This sum
rule is simplified at low density to a new relation with the in-medium quark
condensate <bar{q}q>*, in-medium pion decay constant F_{pi}^t and in-medium
pion wave-function renormalization Z_{pi}*. Calculating Z_{pi}*at low density
from the iso-scalar pion-nucleon scattering data and relating F_{pi}^t to the
isovector pion-nucleus scattering length b_1^*, it is concluded that the
enhanced repulsion of the s-wave isovector pion-nucleus interaction observed in
the deeply bound pionic atoms directly implies the reduction of the in-medium
quark condensate. The knowledge of the in-medium pion mass m_{pi}* is not
necessary to reach this conclusion.
