PURPOSE: To improve the lack of uniformity of sound field and to increase the accuracy of prober, by covering the irregular oscillation section due to folded electrodes for the piezoelectric oscillator providing electrodes on both the sides by means of buffle plate.

CONSTITUTION: A piezoelectric oscillator 1 provides electrodes on both the sides and a folded electrode is given at a part so that two leads can be led from one side. This piezoelectric oscillator 1 is located on a damper member 13 and fixed in a case 14 with a bonding member 12. Further, on the piezoelectric oscillator 1, a buffle plate 10 made of a rigid body or sound shield member is provided so that sound field irregular part due to the said folded electrode can be covered. This buffle plate 10 covers the symmetrical position with irregular oscillation part to avoid the center position of sound field from being shifted.