  The transverse magnetoresistance of the Bechgaard salt (TMTSF)$_2$PF$_6$ has
been measured for various pressures, with the field up to 24 T parallel to the
lowest conductivity direction c$^{\ast}$. A quadratic behavior is observed in
the magnetic field dependence of the spin-density-wave (SDW) transition
temperature $T_{\rm {SDW}}$. With increasing pressure, $T_{\rm {SDW}}$
decreases and the coefficient of the quadratic term increases. These results
are consistent with the prediction of the mean-field theory based on the
nesting of the quasi one-dimensional Fermi surface. Using a mean field theory,
$T_{\rm {SDW}}$ for the perfect nesting case is estimated as about 16 K. This
means that even at ambient pressure where $T_{\rm {SDW}}$ is 12 K, the SDW
phase of (TMTSF)$_2$PF$_6$ is substantially suppressed by the
two-dimensionality of the system.
