In an optical element aimed at readily and accurately controlling a refractive index of an electrooptic effect film, and at making the device adaptive to further downsizing, a voltage control unit controls the refractive index of light in an optical waveguide between two values of a first voltage and a second voltage bounded at a predetermined voltage which corresponds to an anti-ferroelectric phase transition point, based on a fact that the refractive index of a core layer of the optical waveguide largely varies in a digital manner at the predetermined voltage, but is kept almost constant thereunder and thereabove.