A catheter is provided which allows medical treatment of the vascular aneurysm to be carried out with certainty and safely. The catheter includes a catheter main body including a first lumen for supplying and discharging expanding fluid therethrough and a second lumen into which an elongated instrument for the medical treatment is fitted, a balloon provided at a distal end portion of the catheter main body and communicating with the first lumen such that the balloon is expanded and contracted by supply and discharge of the expanding fluid thereto and therefrom, respectively, and a communication section open to an outer face of the balloon, communicating with the second lumen, and isolated from the inside of the balloon.
