  The magnetic field dependence of the SDW transition in (TMTSF)$_2$ClO$_4$ for
various anion cooling rates has been measured, with the field up to 27T
parallel to the lowest conductivity direction $c^{\ast}$. For quenched
(TMTSF)$_2$ClO$_4$, the SDW transition temperature $T_{\rm {SDW}}$ increases
from 4.5K in zero field up to 8.4K at 27T. A quadratic behavior is observed
below 18T, followed by a saturation behavior. These results are consistent with
the prediction of the mean-field theory. From these behaviors, $T_{\rm {SDW}}$
is estimated as $T_{\rm {SDW_0}}$=13.5K for the perfect nesting case. This
indicates that the SDW phase in quenched (TMTSF)$_2$ClO$_4$, where $T_{\rm
{SDW}}$ is less than 6K, is strongly suppressed by the two-dimensionality of
the system. In the intermediate cooled state in which the SDW phase does not
appear in zero field, the transition temperature for the field-induced SDW
shows a quadratic behavior above 12T and there is no saturation behavior even
at 27T, in contrast to the FISDW phase in the relaxed state. This behavior can
probably be attributed to the difference of the dimerized gap due to anion
ordering.
