  One of the advantages of the X-ray Imaging Spectrometer (XIS) system on board
Suzaku is its low and stable non-X-ray background (NXB). In order to make the
best use of this advantage, modeling the NXB spectra with high accuracy is
important to subtract them from the spectra of on-source observations. We
construct an NXB database by collecting XIS events when the dark Earth covers
the XIS FOV. The total exposure time of the NXB data is about 785 ks for each
XIS. It is found that the count rate of the NXB anti-correlates with the
cut-off-rigidity and correlates with the count rate of the PIN upper
discriminator (PIN-UD) in Hard X-ray Detector on board Suzaku. We thus model
the NXB spectrum for a given on-source observation by employing either of these
parameters and obtain a better reproducibility of the NXB for the model with
PIN-UD than that with the cut-off-rigidity. The reproducibility of the NXB
model with PIN-UD is 4.55-5.63% for each XIS NXB in the 1-7 keV band and
2.79-4.36% for each XIS NXB in the 5-12 keV band for each 5 ks exposure of the
NXB data. This NXB reproducibility is much smaller than the spatial fluctuation
of the cosmic X-ray background in the 1-7 keV band, and is almost comparable to
that in the 5-12 keV band.
