A treatment tool includes a second sheath (12) having a slit (12e) through which a guide wire is configured to be taken in and out of a first lumen, and a sheath-retaining part having a guide wire insertion opening (20b) having a slit (20c) and a main opening (20d), wherein a first end portion of the second sheath (12) has a first side cut surface (12g) and a second side cut surface (12h) intersecting in an X-shape, an axially cut surface formed by cutting in parallel to a central axis (O12) from an end surface (12k), an opening surrounded by the first side cut surface (12g), the second side cut surface (12h), and the axially cut surface and communicating with the first lumen at a position overlapping the main opening (20d), and a piece-like part.
