  The effects of nonmagnetic impurity on the spin excitation spectra in a
$d_{x^2-y^2}$-wave superconductor are examined, using the self-consistent
$t$-matrix approximation. It is shown that the impurity self-energy acts to
shift the position of the resonance peak to low frequencies and broaden the
peak. While the impurity vertex correction causes a broad spectral weight in
the spin gap at the impurity concentrations where no clear resonance peak is
observed. The gaplike feature still remains in low frequency region upon the
introduction of impurities. Incorporating these two effects, we find that the
result is in qualitative agreement with experiments on
YBa_{2}(Cu_{1-x}Zn_{x})_{3}O_{6+y}.

Superconductivity