CeB6, a typical Gamma_8-quartet system, exhibits a mysterious antiferroquadrupolar ordered phase in magnetic fields, which is considered as originating from the T_{xyz}-type magnetic octupole moment induced by the field. By resonant x-ray diffraction in magnetic fields, we have verified that the T_{xyz}-type octupole is indeed induced in the 4f-orbital of Ce with a propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed an asymmetric field dependence of the intensity for an electric quadrupole (E2) resonance when the field was reversed, and extracted a field dependence of the octupole by utilizing the interference with an electric dipole (E1) resonance. The result is in good agreement with that of the NMR-line splitting, which reflects the transferred hyperfine field at the Boron nucleus from the anisotropic spin distribution of Ce with an O_{xy}-type quadrupole. The field-reversal method used in the present study opens up the possibility of being widely applied to other multipole ordering systems such as NpO2, Ce_{x}La_{1-x}B_{6}, SmRu_{4}P_{12}, and so on.
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