Refractive index, an optical property, and liquid density were measured for 17 types of room-temperature ionic liquids at various temperatures. Ab initio calculations were performed to yield theoretical polarizability of the respective ionic liquid composing ions. A highly linear correlation was found between the experimental refractive index and the predicted polarizability normalized in terms of the molar volume. This suggests that the electronic polarization of the ionic liquid composing ions predominantly contributes to the refractive index of the room-temperature ionic liquids.