Abstract: A catheter includes: a shaft at least a part of which is to be inserted into a blood vessel; an inlet; an outlet formed in the shaft; a lumen which extends inside the shaft, and through which the inlet and outlet communicate with each other; a balloon disposed on an outer circumference of the shaft, and expandable and contractable; an electrode disposed on a surface of the balloon; a signal acquirer electrically connected to the electrode; a first hole formed in the shaft in a zone between the balloon and the inlet; and a second hole formed in the shaft in a zone between the balloon and the outlet. A blood flowing through the blood vessel is made possible to flow in one of the first and second holes, and flow out from the other hole.