A method of driving an optical modulation device includes applying a voltage to an upper electrode; forming a forward phase slope by applying a first driving signal to a lower electrode in a first area; forming a backward phase slope by applying a second driving signal different from the first driving signal to another lower electrode in a second area; forming an area without phase retardation by applying a third driving signal to two lower electrodes in a third area between the first area and the second area; and forming a flat phase slope by applying a fourth driving signal different from the first to third driving signals one lower electrode in a fourth area between the first area and the second area.