Highly radiopure NaI(Tl) crystal has been developed to investigate cosmic dark matter. The dark matter problem is one of the essential subjects in particle physics and cosmology. Despite the many null results in the dark matter search, one group (DAMA/LIBRA) reported a significant annual modulation in the energy spectrum, which can be interpreted as the dark matter signal. This unknown signal must be a purer NaI(Tl) crystal. The PICOLON project has developed a highly radiopure NaI(Tl) crystal. They purified the NaI(Tl) crystal with combined methods; re-crystallization and resin. The 40K were effectively removed from the NaI by the re-crystallization method. On the other hand, they successfully removed 210Pb by selecting the appropriate resin. The concentration of radioactive impurities in their NaI(Tl) crystal reached below their goals.