A method and a medical elongate body are configured to prevent stagnation or turbulence of blood flow in a recess of a rugged pattern formed in a blood vessel due to bulging of a blood vessel wall at a lesion part of the blood vessel. The method involves partitioning an inside of the blood vessel into upstream and downstream sides of the recess, and introducing gel into the recess to at least partially fill the recess. A blood vessel lumen forming method and medical elongate body to form such a lumen are other aspects of the disclosure and involve introducing gel into the recess to at least partially fill the recess with the gel, and drilling the gel to remove at least some of the gel to form a passage and secure blood flow in the blood vessel.