Disclosed is a treatment device for occluding a body lumen, including: an elongated body which can be inserted into the body lumen； a flat portion forming section which is supported by the elongated body, is expandable widthwise on a more distal side than the elongated body, and deforms the body lumen into a form having a flat portion at a distal portion of the flat portion forming section upon the expansion of the flat portion forming section within the body lumen； and an administering section which administers, toward the flat portion formed by the flat portion forming section, a treatment which acts to occlude the flat portion.
