PURPOSE: To eliminate a local temp. rise and to obviate the generation of degradation in characteristics even in longterm continuous driving by providing the joint surfaces of an acoustooptical medium and a piezoelectric diaphragm with a specific inclination when the opposite surfaces are viewed from a progressing direction of a laser beam.

CONSTITUTION: A piezoelectric diaphragm section 120 consists of a first electrode plate 122 joined to the diaphragm 121 and acoustooptical medium 110 side and a second electrode plate 123 joined to the surface facing this plate. The angle θ1 of inclination when the surface opposite to the joint surface of the piezoelectric diaphragm is viewed from the progressing direction of the laser beam is set at 33 to 37° (slope 140). Ultrasonic waves reflect 8 times at the flanks parallel with the progressing direction of the laser beam of the acoustooptical medium 110 after the ultrasonic waves reflect on the surface facing the joint surface of the piezoelectric diaphragm and before the waves return to the joint surface. The ultrasonic waves are absorbed at the reflection surface and the intensity thereof is attenuated by a half or below at every time. The greater part of the energy is absorbed before the waves arrive at the joint surface of the piezoelectric diaphragm. The generation of the heat in the joint surface of the piezoelectric diaphragm is, therefore, decreased.