A theoretical method is developed for examining the intermolecular vibrational motions that are responsible for modulating the external electric field at the position of the solute in solution. The derivation is done in the context of the instantaneous normal mode theory by using an algebraic property of the interaction formula, in the same way as in the intensity-carrying mode theory of intramolecular vibrations developed in our previous studies. The vibrational motions derived by the present method are called field-modulating modes (FMMs). These modes play an essential role in electrostatic intermolecular vibrational interactions in solution. As an example of application of the present method, the case of an acetonitrile solution of the nitrate ion (NO3-) is studied. Various characteristics of the FMMs, which are important for vibrational polarization of the solute, are examined in detail. The relation of the present picture to vibrational relaxation theories is also discussed.