Many biomolecules have metal atoms playing important role in the biological activity. To reveal the mechanism of the biological activity at molecular level, the classical mechanical treatment is insufficient because of the flexible electronic states of metal atoms, and the quantum mechanical treatment is indispensable. The fragment molecular orbital (FMO) method treats the electronic wave-function of biomolecule explicitly, and carries out a massive molecular orbital calculation with convenient computational cost. We carried out the FMO calculation of the DNA and estrogen receptor (ER) complex, and reveal the detail interaction and the change of the ER electronic state induced by DNA binding.