  We study a Hubbard model with a strong onsite impurity potential based on an
improved fluctuation-exchange (FLEX) approximation, which we call the GVI-FLEX
method. We find that (i) both local and staggered susceptibilities are strongly
enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime
as well as the local density of states (DOS) are strongly suppressed in a wide
area around the impurity (like a Swiss cheese hole), which causes the ``huge
residual resistivity'' beyond the s-wave unitary scattering value. These
results by the GVI method naturally explains the various impurity effects in
HTSC's in a unified way, which had been a long-standing theoretical problem.
