An ultrasonic vibrator and an ultrasonic treatment device which can adjust a resonant frequency while maintaining driving force are provided. An ultrasonic vibrator 1 includes two metal blocks 2, a driving unit 3 that is arranged between the metal blocks 2 and produces a piezoelectric effect to vibrate by application of an alternating voltage, and at least one adjustment unit 4 that is arranged between the metal blocks 2 and the driving unit 3 in an insulated state and changes Young's modulus.