PURPOSE: To suppress influence of dirt, dust, etc. on the incident and exit faces of an ultrasonic optical modulating element by reducing the diameter of the beam incident on said element only in the propagating direction of an ultrasonic wave.

CONSTITUTION: A beam compressor 6 and a beam expander 7 are formed of a pair of cylindrical lenses having refracting power only within the progressing direction of an ultrasonic wave. Only the beam diameter dH in the propagating direction of the ultrasonic wave incident on an ultrasonic optical modulating element 2 is reduced by such lenses. Since the modulating optical system is formed in such a way, the beam diameter dH in the propagating direction of the ultrasonic wave in the element 2 is so reduced as to have the access time meeting a modulating band and on the other hand the beam diameter dV in the vertical direction is increased. The influence of dirt, dust, etc. on the incident and exit faces of the modulating element is thus suppressed.