This element consists of a ultrasonic wave solid propagation medium (1), input (4) and output (6) transducers and backing members (A,B). The ultrasonic wave solid propagation medium (1) has a ultrasonic wave propagation distance (S₁) which is shorter than a distance (S₃) between main surfaces or a distance (S₂) between side sur­faces. The input and output transducers (4,6) attached to ultrasonic wave signal input and output surfaces (15,16) of the foregoing propagation medium (1) serve convert an input electric signal into a ultrasonic wave and and to convert this ultrasonic wave into an output electric signal. The backing members (A,B) are joined to surfaces (17,18) opposite to joint surfaces (15,16) of the input and output transducers (4,6) to the propagation medium (1).