PURPOSE: To detect a weak signal, by forming a lens to one end of a columnar sensor and forming the other end of said sensor into a spherical surface and providing a plurality of piezoelectric elements to said spherical surface at the positions corresponding to the incident and reflected angles with respect to an object to be observed.
CONSTITUTION: A piezoelectric element forming surface 4 of a sensor main body 7 is formed into a spherical surface having the focus Q of the lens 8 of said sensor main body 7 as the center of a sphere. A piezoelectric element 1 is provided on the central part of the piezoelectric element forming surface 4 and piezoelectric elements 2, 3 are formed at the symmetric positions around said piezoelectric element 1, that is, at the positions where angles θ 1 , θ 2 are equal with each other. Then, a high frequency wave is transmitted from a high frequency transmitter 6 and ultrasonic waves are propagated through the sensor main body 7 from the piezoelectric elements 1W3. The ultrasonic wave from the piezoelectric element 1 reaches an object 9 to be observed in the vicinity of the focus Q as shown by an arrow (a) to be reflected as shown by an arrow a'. The respective ultrasonic waves from the piezoelectric elements 2, 3 reach the object 9 to be observed as shown by arrows (b), (c) to be reflected as shown by arrows b', c'. Since the ultrasonic waves are received from the object to be observed by a plurality of the piezoelectric elements, even a material having high ultrasonic wave absorptivity can be well observed.