  The new Super-Kamiokande-Gadolinium (SK-Gd) project is an upgrade of the
Super-Kamiokande (SK) detector. In the SK-Gd project, 0.2\% Gd$_2$(SO$_4$)$_3$
is loaded into the 50 kton water tank of the SK. One of the main purposes of
the project is to discover Supernova Relic Neutrinos. Neutrino measurements and
proton decay searches will also be performed in the SK-Gd. In order to measure
solar neutrinos with a low energy threshold of $\sim$3.5 MeV in the SK-Gd, the
main radioactive contaminations, $^{238}$U and $^{232}$Th, in
Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O, should be minimized before loading. Our
maximum levels for U and Th are 5 mBq (U)/kg (Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O)
and 0.05 mBq (Th)/kg (Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O).
  In order to measure such low concentrations of U and Th in
Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O, we developed the solid-phase extraction
technique. Using this method, about 90\% or more U and Th could be efficiently
extracted while Gd was reduced by a factor of about $10^{4}$. This allowed
these radioactivity contaminations to be measured precisely as 0.04 mBq/kg
(Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O) for U and 0.01 mBq/kg
(Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O) for Th. We measured three pure
Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O samples using this method and estimated that
the purest one contained $<0.04$ mBq (U)/kg (Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O)
and 0.06 $\pm$ 0.01 mBq (Th)/kg (Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O) by ICP-MS.
