An introducer is configured to distinguish an introducer provided with a sheath tube whose wall thickness is reduced, from an introducer provided with a sheath tube whose wall thickness is not reduced, and to recognize the relationship between the inner diameter and the outer diameter for the sheath tube in which the wall thickness is reduced. The introducer includes an introducer sheath and a three-way stopcock portion. The introducer further includes a first information piece on at least one of the introducer sheath and the three-way stopcock portion and has an inner diameter dimension of the sheath tube as an index. Furthermore, a second information piece having an outer diameter dimension of the sheath tube as an index is on at least one of the introducer sheath and the three-way stopcock portion.