A piezoelectric device is attached to an inner bottom surface of the outer case having a tubular shape including a bottom, and an inner case is disposed within the outer case. In an ultrasonic vibration acting surface of the inner case that is arranged to face the bottom surface of the outer case, a mass of the inner case is arranged to restrain vibration of the outer case, which is generated by the piezoelectric device. A first cutout is provided in a portion of the ultrasonic vibration acting surface and arranged to face the piezoelectric device so as to flatten an ultrasonic beam generated by vibrations of the piezoelectric device and the outer case. Second cutouts are provided at locations on the ultrasonic vibration acting surface spaced away from the first cutout in a line symmetrical relationship with a long axis of the first cutout defining a symmetrical axis.