  Understanding a background is crucial in particular for a study of low
surface brightness objects. In order to establish the background subtraction
method, we have studied properties of the EPIC background.
  Count rates of the background vary violently by two order of magnitude at the
maximum, while during the most quiet period, these are stable within 8 % at a 1
$\sigma$ level. The overall spectrum is dominated by particle events above 5
keV, and its spatial variation is also found.
  The long-term variation of the background is also investigated with CAL
CLOSED data, which is the data of calibration source with filter closed. The
average background count rate decreased by 20 % from March 2000 to January
2001, but it regained in February 2001.
  For the modeling of the background spectrum, we investigate correlations
between the 2-10 keV count rate and some characteristic parameters. The PN
background shows a good correlation with some parameters. On the other hands,
the MOS background does not shows a clear correlation. Further investigation is
needed for the MOS background.
  Our final goal is to establish a method to predict the background, for which
these results will be reflected in the background generator.
