The splitting of the spin-isospin dipole resonance, which has been observed in the recent measurements of the cross section for electro-excitation of 12C, is discussed together with the splitting of the isospin dipole resonance on the basis of the deformed nuclear model. The obtained distribution of the isospin and spin-isospin excitation strength over the giant resonance region is in good agreement with experiment. In particular, it is found that the strong spin-isospin transition should appear in the same energy region (≈ 23 MeV) as that of the isospin mode. This result provides a reasonable fit with the cross section for electron scattering from 12C observed at backward angles with high-momentum transfer which could not be explained by previous studies.