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 forms the body lumen with a flat portion at a distal portion of the flat portion forming section attendant on the expansion of the flat portion forming section within the body lumen； and a giving section which gives, toward the flat portion formed by the flat portion forming section, that which acts to occlude the flat portion.
