  There exist model calculations showing the modification of the hadronic
parameters of $\rho$ meson in the nuclear environment. From these parameters,
we extract the $\rho$ meson nucleus optical potential and show the medium
effect due to this potential on the $\rho$ meson mass distribution spectra in
the photonuclear reaction. The calculated results reproduced reasonably the
measured $e^+e^-$ invariant mass, i.e., $\rho$ meson mass, distribution spectra
in the $( \gamma, \rho^0 \to e^+e^- )$ reaction on nuclei.
