By an electro-optical switch or an optical switch element utilizing diffraction by a surface acoustic wave, an input light beam is first switched over to one of light beams two or a multiple of two in number and propagating in different directions. By a surface acoustic wave which is discretely variable in frequency, the light beam in each of the different propagation direction is deflected at angles in accordance with the varying frequencies while propagating through an optical waveguide layer. It is possible to obtain by switching a very large total number of output light beams which are equal in number to the number of resolvable deflected light beams obtained in the frequency bandwidth of one surface acoustic wave, multiplied by the number of light beams obtained by switching with the optical switch element.