  The highly radiopure NaI(Tl) was developed to search for particle candidates
of dark matter. The optimized methods were combined to reduce various
radioactive impurities. The $^{40}$K was effectively reduced by the
re-crystallization method. The progenies of the decay chains of uranium and
thorium were reduced by appropriate resins. The concentration of natural
potassium in NaI(Tl) crystal was reduced down to 20 ppb. Concentrations of
alpha-ray emitters were successfully reduced by appropriate selection of resin.
The present concentration of thorium series and 226Ra were $1.2 \pm1.4$
$\mu$Bq/kg and $13\pm4$ $\mu$Bq/kg, respectively. No significant excess in the
concentration of $^{210}$Pb was obtained, and the upper limit was 5.7
$\mu$Bq/kg at 90% C. L. The achieved level of radiopurity of NaI(Tl) crystals
makes construction of a dark matter detector possible.
