The present disclosure relates to a transmittance-variable device, a driving method thereof, a method for improving a light shielding ratio, and a use thereof. In some embodiments, a transmittance-variable device includes a transmittance-variable film capable of being switched between a transparent mode and a black mode depending on application of a voltage signal, and a power source for applying a voltage signal such that the intensity of the voltage signal decreases with time to implement the black mode, wherein the transmittance-variable film includes a first electrode substrate, an electrophoretic layer, and a second electrode substrate sequentially arranged. The transmittance-variable device can exhibit an excellent light shielding ratio in the black mode after driving with a voltage signal, and such a transmittance-variable device can be usefully used in a smart window.