The general dynamic aspect of electron density of a molecular system is studied on the basis of the general equation of the electron orbital which is formulated for the dynamic study of electronic motion. The newly defined electron orbital incorporates the dynamics of molecular vibration into the electronic structures. In this scheme, the change of electron distribution caused by excitation of vibrational state is defined as the ``dynamic electron transfer.'' The dynamic electron density is found to have the remarkable ``additive'' property. The time-dependent aspect of the dynamic electron redistribution is also analyzed on the basis of the ``coherent state.'' The new method relates the classical vibrational amplitude to the quantum number of the vibrational state. As a preliminary application of the present treatment, the dynamic electron densities of H2, HD, HT, HF, and HCl molecules are calculated by use of ab initio molecular orbital method.