PURPOSE: To increase the impedance to the fundamental wave vibration while the impedance to high frequency vibrator is being suppressed small by approaching a part of the profile of a diaphragm to electrodes.

CONSTITUTION: Circular electrodes 12,13 are provided at the center of both faces of a crystal diaphragm 11 and lead wires 14,15 connected respectively to the electrodes 12,13 are held by a stem 16. In this case, the corner of the diaphragm 11 is cut obliquely by a length (l) and the part of profile of the diaphragm ll is made close to the electrodes 12,13. Then the impedance to the fundamental wave vibration is remarkably increased while the impedance to high frequency vibration is suppressed lower.