  The XMASS detector is a large single phase liquid Xenon scintillator.After
its feasibility had been studied using a 100 kg size prototype detector, an 800
kg size detector is being built for dark matter search with the sensitivity of
$10^{-45} {\rm cm}^2$ region in spin-independent cross section. The results of
R\&D study for 800 kg detector, especially ultra low background technologies,
and the prospects of the experiment are described.
