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 which includes an elongated hollow sheath tube and a sheath hub at the proximal side of the sheath tube and provided with a dilator insertion port. A first information piece having an inner diameter dimension of the sheath tube as an index is shown on at least one of the sheath tube and the sheath hub. Furthermore, a second information piece having an outer diameter dimension of the sheath tube as an index is shown on at least one of the sheath tube and the sheath hub.