Ultrasonic vibration transmittable probe and ultrasonic treatment assembly

An ultrasonic vibration transmittable probe includes a probe body configured to transmit ultrasonic vibration generated by an ultrasonic transducer. A treatment section is provided on a distal end side of the probe body along its longitudinal axis and is configured to cut a treatment object with the ultrasonic vibration. The treatment section includes first to third cutting surfaces disposed at progressively proximal positions. A portion of the first cutting surface has a dimension along a first orthogonal direction orthogonal to the longitudinal axis that is smaller than a dimension of the second cutting surface along the first orthogonal direction.