The 5d transition metal oxide Cd2Os2O7 having a pyrochlore lattice of Os atoms exhibits a metal-insulator transition with a magnetic ordering at 227 K, but the magnetic structure in the ordered state and the origin of the transition have not yet been specified. The commensurate magnetic reflection with the propagation vector k = (0, 0, 0) was first observed in the resonant X-ray scattering using a high-quality single crystal. We also clarified the only one kind of magnetic moment via the synchrotron radiation-based Mossbauer effect. We propose that a strong spin-orbit interaction is notably apparent in the spin arrangement and the insulating mechanism in this compound.