The azimuthal angle dependence observed in the resonant X-ray scattering in phase IV of Ce0.7La0.3B6 is analyzed theoretically. It is shown that the peculiar angle dependence observed in the E2 channel is consistent with the Λ5u-type octupole order with principal axis along  and equivalent directions. Under the assumption that the four equivalent octupole domains are nearly equally populated in the sample, the observed angle dependences are reproduced by calculation for both ωω′ and ωπ′ polarizations. The calculation for various symmetries of order parameters unambiguously excludes order parameters other than the Λ5u-type octupole.
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