  We report a measurement of the diffractive structure function $F_{jj}^D$ of
the antiproton obtained from a study of dijet events produced in association
with a leading antiproton in $\bar pp$ collisions at $\sqrt s=630$ GeV at the
Fermilab Tevatron. The ratio of $F_{jj}^D$ at $\sqrt s=630$ GeV to $F_{jj}^D$
obtained from a similar measurement at $\sqrt s=1800$ GeV is compared with
expectations from QCD factorization and with theoretical predictions. We also
report a measurement of the $\xi$ ($x$-Pomeron) and $\beta$ ($x$ of parton in
Pomeron) dependence of $F_{jj}^D$ at $\sqrt s=1800$ GeV. In the region
$0.035<\xi<0.095$, $|t|<1$ GeV$^2$ and $\beta<0.5$, $F_{jj}^D(\beta,\xi)$ is
found to be of the form $\beta^{-1.0\pm 0.1} \xi^{-0.9\pm 0.1}$, which obeys
$\beta$-$\xi$ factorization.
