Electric excitation of long conjugated chain-molecule is studied by Tomonaga's method of collective motion. One of the most elementary modes of collective motion is the dipole oscillation. Assuming the occurrence of this oscillation for pi-electrons in the conjugated chain, it is found that, for cyanine dyes which contain C-C bonds of equal length, we have the wavelength of absorption which increases nearly in proportion to their chain length, while for carotenoids which contain alternating bond lengths, we have the wavelength of absorption which approaches a finite value with increasing chain lengths of carotenoids. When the classical polarizability is introduced, we have the lower energies of excitation by a little amount. It is found that the introduction of the classical polarizability or the effect of depolarization corresponds to the mixing of configurations.