[Object] To provide a catheter having a simple structure capable of excising an atheroma in a blood vessel and capable of obtaining ultrasonic images of a blood vessel.[Solution] A catheter 10 has a shaft 11 having an opening 20 in a part of the side wall on the distal end side, a cutter 12 which is located in the vicinity of the opening 20 in the internal space of the shaft 11 and which can move in the axial direction 101 of the shaft 11, a balloon 23 which is disposed on the side opposite to the opening 20 with respect to the axis of the shaft 11 and which outwardly expands from the side wall of the shaft 11, and a phased array ultrasound probe 17 disposed along the circumferential direction 102 of the outer peripheral surface of the side wall in the vicinity of the opening 20 at least on the same side as the side where the opening 20 is provided with respect to the axis of the shaft 11.