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.
