PURPOSE: To form an element with a single plate of a Bi12SiO20 single crystal and to enable operation at low voltage by holding the (110) face of the single crystal between a pair of electrodes and applying an electric field in the (110) direction.

CONSTITUTION: Assuming an incident light beam to be a laser beam having wavelength λ=663nm in the equatio, when phase difference T between an ordinary ray and an extraordinary ray passed through an electrooptical element 1 becomes T=π, the plate of a Bi12SiO20 single crystal is regulated to d=1mm and l=10mm as standard sizes. Since the γ41 of the plate is 5/1010cm.V, and the refractive index n0 to light having 633nm wavelength is 2.53, the intensity of the electric field is 3.91kV/cm. Accordingly, voltage for satisfying the condition of phase difference T=n is 391V. By allowing a laser beam having 633nm wavelength to enter an optical switch device as a light beam L and by applying 391V signal voltage between the electrodes, emergent light which is intermittent in accordance with the application of the signal voltage can be obtd.