  Impurity nuclear spin relaxation is studied theoretically. A single impurity
generates a bound state localized around the impurity atom in unconventional
superconductors. With increasing impurity potential, the relaxation rate
$T_1^{-1}$ is reduced by the impurity potential. However, it has a peak at low
temperatures due to the impurity bound state. The peak disappears at
non-impurity sites. The impurity site NMR measurement detecting a local
electronic structure just on the impurity atom is very useful for identifying
the unconventional pairing states.
