Since split electrodes are disposed along the x-axis of an AT cut quartz crystal plate on its ultrathin vibratory portion formed integrally with a thick frame-like marginal portion surrounding it, desired acoustic coupling can be obtained although the split electrodes are disposed closely to such an extent as not to cause shorting between them. It is therefore possible to obtain, with a satisfactory yield, filter elements which have center frequencies in the range greater than about ten megahertz and sufficiently wide passbands.