A probe, which is a transrectal ultrasonic probe, includes an insertion unit to be inserted into the rectum of a subject. The insertion unit includes a shaft in a slim shape closer to the proximal side, and an insertion tip portion that is connected to the distal end of the shaft and has an acoustic head that transmits and receives ultrasonic waves. The ultrasonic waves are transmitted from the acoustic head laterally to the insertion unit. The extension direction of the shaft from the proximal side toward the distal side is curved in the direction opposite to the direction D in which the acoustic head transmits the ultrasonic waves.