PURPOSE: To prevent generation of a reflected wave of an ultrasonic wave in the boundary of an ultrasonic medium and a macromolecular piezoelectric body, by lining the reverse side of the marcomolecular piezoelectric body by use of an acoustical absorbent by which the acoustic impedance is varied stepwise or continuously in the direction of thickness.

CONSTITUTION: Rubber plates 21∼25 having different content percentage of metallic powder, respectively, are provided, and are constituted so that the rubber plate 21 has the minimum and the rubber plate 25 has the maximum content percentage. These plates 21∼25 are laminated and are stuck to each other with an adhesive. On the side having a minimum metallic content, among the laminated rubber plates which have been prepared in this way is stuck a macromolecular piezoelectric element 1. The other surface of the macromolecular piezoelectric element is joined with an ultrasonic medium 2. In this way, generation of a reflected wave of an ultrasonic wave is prevented in the boundary of the ultrasonic medium 2 and the macromolecular piezoelectric element 1.