We have carried out normal mode analysis for four B-DNA molecules with different lengths and sequences. By focusing our attention on motions of the helical axes of the molecules and comparing them with those of the ideal rod obeying vibrational theory, we have shown that DNA molecules behave like isotropic and homogeneous elastic rods. Then, we have calculated dynamic constants representing their rigidities for bending and twisting. In the analysis of their twisting motions, we have shown that sliding and shifting motions between adjacent bases are important for the motions.