A two-impurity problem in lattice vibrations is investigated with particular attention to that in-band mode which becomes a localized mode of lower frequency when two impurities are well separated with each other. The cosine transform of the propagator of the classical motion of one of the impurities is calculated as an application of the results obtained in a preceding paper, and on the other hand the optical absorption by the impurities is studied by supposing that they are charged while stoms of the host crystal are uncharged. By taking a simple cubic lattice as a model, it is shown that this in-band mode gives rise to a sharp peak in the high frequency region when the cosine transform or the absorption coefficient is plotted as a function of frequency. Such a sharp peak in the in-band region can be considered as being due to a resonant mode which is quasi-localized around two impurity atoms. A discussion is also given of a high-frequency resonant mode in one-and few-impurity problems.