Dioxygen binding process of nonheme iron(III) center in intradiol catechol dioxygenase was investigated with CASSCF/CASPT2 method to incorporate multiconfigurational character participating in Fe-O-2 interaction. In this process, two alternative mechanisms were proposed: one is called "oxygen activation" and the other is called "substrate activation". Our CASSCF/CASPT2-calculated results support the oxygen activation. Potential energy curves and electronic structure evaluated with SA(state-averaged)-CASSCF/CASPT2 method indicate that the charge transfer directly occurs from the catecholate moiety to the dioxygen moiety in the O-2 binding process, to produce eta(1)-end-on type iron(III)-superoxo complex. This is the key step of the dioxygen activation. Interestingly, the iron center always keeps high spin d(5) character during the O-2 binding process, indicating the iron(III) center does not receive charge transfer from the catecholate moiety. However, this does not mean that the iron(HI) center is not necessary to the dioxygen activation. The important role which the iron(III) center plays in catechol dioxygenase is to adjust the energy level of O-2 to induce the charge transfer from the catecholate moiety to the dioxygen moiety. Besides the eta(1)-end-on iron(III)-superoxo complex, eta(2)-side-on type iron(III)-superoxo complex is also optimized. This species is more stable than the eta(1)-end-on type iron (III)-superoxo complex, suggesting that this is considered as a stable isomer in the early stage of the catalytic cycle.