We investigate how the second-order mode-mode interaction in the excited state affects an intensity distribution of optical absorption lines, by calculating the Frank-Condon factor exactly. As a result, we find that the mode-mode interaction affects a general form of the intensity distribution of optical absorption spectra mainly through displacement parameters in the excited state. Furthermore, we find that the line width of each absorption line is affected very much by the mode-mode interaction. In this study fourteen species of elementary transition operators which annihilate, create and transfer some vibrational quanta in and between two modes are defined. We find that coefficients in the formula of the Frank-Condon factor have physical meanings of transition-probability amplitudes corresponding to elementary transition operators. An interference phenomenon in the transition is discussed.