  Effects of nonmagnetic impurities on optical conductivity in the systems of
antiferromagnetically correlated electrons are examined based on the Lanczos
exact diagonalization scheme. As a result of resonant scattering a
low-frequency peak in the optical conductivity is predicted to occur in the
presence of the nonmagnetic impurities, which is consistent with the observed
normal-state optical conductivity of ${\rm YBa_2}({\rm Cu}_{1-x}{\rm
Zn}_x)_3{\rm O}_{7-\delta}$. In addition, a relatively high and broad peak is
found to occur at a high frequency region as a consequence of the Heisenberg
interaction between electrons, in agreement with observation in the peak
position.
