A transmission unit of an ultrasonic probe includes a groove defining portion defining a groove portion, depressed from an unit outer surface toward a first perpendicular direction perpendicular to a longitudinal axis and open toward a second perpendicular direction opposite to the first perpendicular direction, over an entire length of the transmission unit in axially parallel directions parallel to the longitudinal axis, and a sectional area changing portion changing a sectional area perpendicular to the longitudinal axis in the transmission unit. A gravity center position in a section perpendicular to the longitudinal axis in the case where the longitudinal axis is the reference position is consistent over the entire length of the transmission unit in the axially parallel directions.
