Polarization inversion domains 2 are irregularly formed in a ferroelectric substrate 3 constituting an electro-optical element 1 at least in the propagation direction of a light beam A. An A.C. current signal is applied between electrodes 4 and 5. It is thereby arranged to disturb the phase of each of the light rays constituting a light beam whose coherent characteristic is high, such as a laser light or the like, and thereby to easily obtain the light beam whose coherent characteristic has been decreased.