A treatment method is disclosed, which is capable of causing a medical elongated body to indwell the inside of a palmar artery of a patient, while suppressing kink, poor torque transmission ability, and poor operability of the medical elongated body. The treatment method includes causing a sheath tube of an introducer introduced into a blood vessel via a puncture site formed on an ulnar artery side of a palmar artery of the patient, to indwell the puncture site.