Provided is an ultrasonic probe which is capable of reliably measuring a blood vessel diameter without being influenced by the position of the ultrasonic probe when placed on the surface of the skin. This ultrasonic probe (P) for measuring a diameter of a blood vessel (B) of a living subject using the pulse-echo method comprises an oscillating unit (2) to which a pulse voltage is applied such that the oscillating unit (2) generates a pulse wave toward the blood vessel, and a reception unit (3) which receives a reflected wave that impinges on and is reflected by the blood vessel. The oscillating unit comprises a plurality of piezoelectric elements (22) which are regular hexagonal in plan view arranged in a honeycomb in the same plane, wherein the pulse voltage is selectively applied to each of the piezoelectric elements by a pulsed power supply (E).