  It is investigated, on the basis of the fluctuation exchange approximation,
how the shape of the Fermi surface is modified in two-dimensional $t-t'-U$
Hubbard model at half filling as strength of the onsite Coulomb interaction $U$
is increased. The antiferromagnetic (AF) correlation length is shown to be
enhanced as the Coulomb interaction get closer to the critical value $U_{\rm
c}$ for the onset AF order. At the same time, the shape of the renormalized
Fermi surface is deformed showing the tendency of nesting near $U_{\rm c}$.
