The anisotropy of the effect of pf mixing on the electronic structure of Ce 1 - x La x Sb(x=0,0.1) in the magnetically ordered states is examined by measuring the magnetic linear dichroism of the optical conductivity spectrum (σ | | (ω) for E||B||[001], σ (ω) for E B||[001]) in the infrared region at low temperatures under magnetic fields. The σ (ω) spectrum changes significantly with magnetic phase, whereas σ | | (ω) changes little. This result indicates that the electronic structure perpendicular to the magnetic field is more strongly modulated by pf mixing than that parallel to the field.