A puncture adapter is provided for being mounted on an ultrasonic probe. The ultrasonic probe to be mounted comprises a rod-like insertion portion that is insertable into a subject and has a search unit for transmitting and receiving ultrasonic waves at the tip end of the insertion portion, and in which a guide channel for guiding a puncture needle is formed on a side surface of the insertion portion. This puncture adapter has a first tubular component on which a gap is provided on a side surface along the length direction, which gap is from one end to the other side of the side surface. The first tubular component is mounted so as to contact the outside of the insertion portion and cover the guide channel, and is turnable in the circumferential direction thereof.