A transmittance-variable device, a driving method thereof, a method for improving a light shielding ratio therein, and a use thereof are disclosed herein. In some embodiments, a transmittance-variable device includes a transmittance-variable film capable of switching between a transparent mode and a black mode depending on application of a voltage signal; and a power source for applying a voltage signal having a frequency of 30 Hz or less to implement the black mode, wherein the transmittance-variable film comprises 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.