  The nesting vector and the magnetic susceptibility of the
quasi-one-dimensional system having imperfectly nested Fermi surface are
studied analytically and numerically. The magnetic susceptibility has the
plateau-like maximum in ``\textit{sweptback}'' region in the momentum space,
which is surrounded by $\mathbf{Q}=(2 k_F, \pi) + \mathbf{q}_i$ ($k_F$ is the
Fermi wave number, $i=1,3,4$, and $\mathbf{q}_1$, $\mathbf{q}_3$ and
$\mathbf{q}_{4}$ are given in this paper). The best nesting vector, at which
the susceptibility $\chi_0(\mathbf{Q})$ has the absolute maximum at T=0, is
obtained near but not at the inflection point, $\mathbf{Q}=(2 k_F,
\pi)+\mathbf{q}_4$. The effect of the periodic potential $V$ on the
susceptibility is studied, which is important for the successive transitions of
the field-induced spin density wave in (TMTSF)$_2$ClO$_4$. We obtain that the
sweptback region (surrounded by $\mathbf{q}_2$, $\mathbf{q}_3$ and
$\mathbf{q}_4$ when $V>0$) becomes small as $V$ increases and it shrinks to
$\mathbf{q}_3$ for $V \geq 4 t_b'$, where $t_b'$ gives the degree of imperfect
nesting of the Fermi surface, i.e. the second harmonics of the warping in the
Fermi surface. The occurrence of the sign reversal of the Hall coefficient in
the field-induced spin density wave states is discussed to be possible only
when $V<2 t_b'-2 t_4$, where $t_4$ is the amplitude of the fourth harmonics of
the warping in the Fermi surface. This gives the novel limitation for the
magnitude of $V$.
