  We investigate the sensitivity of the medium effect in the high-density
region on the nucleus-nucleus elastic scattering in the framework of the
double-folding (DF) model with the complex $G$-matrix interaction. The medium
effect including three-body-force (TBF) effect is investigated with two
methods. In the both methods, the medium effect is clearly seen on the
potential and the elastic cross section. Finally, we make clear the crucial
role of the TBF effect up to $k_F =$ 1.6 fm$^{-1}$ in the nucleus-nucleus
elastic scattering.
