We present the measurement of the diffractive structure function 
of the antiproton in proton-antiproton collisions at the center-of-mass energy
of 1.8 TeV. The data were collected during the 1995-1996 Tevatron 
collider run using the Collider Detector at Fermilab~(CDF).
Diffractive events are characterized by the recoil $\bar{p}$ 
and the forward rapidity gap in the fnal state.
In order to tag the recoil $\bar{p}$, we installed a Roman-pot type 
antiproton spectrometer~(RPS), and using the forward calorimeter and 
the beam-beam counter, we identify the rapidity gap. 
In order to identify a hard collision, we require the presence of two 
or more jets.
In this analysis, we calculate the hard scale in the interaction~($Q^2$),
the momentum fraction of antiproton carried by struck parton~($\xpb$), and
the fractional momentum of the struck parton in the exchanged object~($\beta$).

We obtained the diffractive structure function
as a function of $\beta$ and $Q^2$ in the kinematic region of 
$0.035\leq\xi\leq 0.095$ and $|t|\leq 1$ GeV${}^2$. 
We observe no $Q^2$ dependence
in the diffractive structure function of the antiproton
in the $Q^2$ region measured in this study.
This is consistent with the H1 result at HERA $ep$ collider experiment.
The absolute value of the diffractive structure function measured at CDF
is smaller than that of the H1 result by, approximately, a factor of 1/7.
The discrepancy is also observed in other results at CDF such as 
the diffractive W and b-quark production where the cross section
is about 20\% of the prediction using the Donnachie-Landshoff pomeron flux.
These results suggest that the diffractive cross section is not described 
by using the universal diffractive structure function of the antiproton.