  A dark matter search project needs and extremely low background radiation
detector since the expected event rate of dark matter is less than a few events
in one year in one tonne of the detector mass. The authors developed a highly
radiopure NaI(Tl) crystal to search for dark matter. The best combination of
the purification methods was developed, resulting $^{\mathrm{nat}}$K and
$^{210}$Pb were less than 20 ppb and 5.7 $\mu$Bq/kg, respectively.
  The authors will construct a large volume detector system with high-purity
NaI(Tl) crystals. The design and the performance of the prototype detector
module will be reported in this article.
