A treatment method involves introducing a catheter into a blood vessel in an arm of the patient and is advanced to position the distal end of the catheter in a blood vessel in a lower limb on one side of the patients body. A treatment device is inserted into the catheter and advanced to a treatment target in the lower limb on the one side of the patients body. The catheter is then moved to shift the distal end of the catheter from the blood vessel in the lower limb on the one side of the patients body to a blood vessel in the lower limb on the other side of the patients body. A treatment instrument can then be introduced into the catheter and advanced to carry out treatment on a treatment target in the lower limb on the other side of the patients body.
