  We study resonant x-ray scattering (RXS) at Np M_{4,5} edges in the
triple-\textbf{k} multipole ordering phase in NpO_{2}, on the basis of a
localized electron model. We derive an expression for RXS amplitudes to
characterize the spectra under the assumption that a rotational invariance is
preserved in the intermediate state of scattering process. This assumption is
justified by the fact that energies of the crystal electric field and the
intersite interaction is smaller than the energy of multiplet structures. This
expression is found useful to calculate energy profiles with taking account of
the intra-Coulomb and spin-orbit interactions. Assuming the \Gamma_{8}-quartet
ground state, we construct the triple-\textbf{k} ground state, and analyze the
RXS spectra. The energy profiles are calculated in good agreement with the
experiment, providing a sound basis to previous phenomenological analyses.
