In a width dimension decrease portion forming a distal end of an ultrasonic probe, a width dimension decreases from a proximal direction toward a distal direction, so that a sectional area perpendicular to a longitudinal axis decreases from the proximal direction toward the distal direction. An intermediary portion continuous between a probe body portion and the width dimension decrease portion includes a width dimension increase portion in which the width dimension increases and a thickness dimension decreases from the proximal direction toward the distal direction at the same time. In the intermediary portion, the sectional area perpendicular to the longitudinal axis does not increase and does not decrease at a higher decrease rate than in the width dimension decrease portion toward the distal direction.
