[en] The alignment effect on the L-shell ionization of Nb atom (X=41) by 0.84 MeV, 1.38 MeV and 2.92 MeV proton impact was investigated. The polarization of L_l X-rays emitted from the aligned niobium target was measured with the crystal spectrometer combined with a position sensitive proportional counter. The experimental values of the degree of alignment A₂ were compared with the prediction of PWBA theory. It results that the experimental value of A₂ in 1.38 MeV proton impact was much larger than the theoretical one, and A₂ of 0.84 MeV almost coincides with that of 2.92 MeV. (author)