[Problem] To provide: a dilator that can lessen the resistance in expanding the diameter of an introduction hole felt by an operator at the time of inserting the dilator into the introduction hole, and can thus be inserted smoothly into the introduction hole and an introducer assembly and a medical tool having said dilator. [Solution] This dilator (30) has a first section (311), a second section (312), a third section (313), and a fourth section (314) that have different diameters and are formed adjacent to one another in the axial direction in the aforementioned order from the tip end in the axial direction toward the base end. The angle (α1) that the outer peripheral surface (S1) of the first section forms with respect to the axial direction is smaller than the angle (α2) that the outer peripheral surface (S2) of the second section (312) forms with respect to the axial direction. The angle that the outer peripheral surface of the second section forms with respect to the axial direction is larger than the angle (α3) that the outer peripheral surface (S3) of the third section (313) forms with respect to the axial direction. The angle that the outer peripheral surface of the third section forms with respect to the axial direction is smaller than the angle (α4) that the outer peripheral surface (S4) of the fourth section (314) forms with respect to the axial direction.La présente invention vise à fournir un dilatateur qui puisse amoindrir la résistance rencontrée par un opérateur lors de lélargissement du diamètre dun trou dintroduction au moment dinsérer le dilatateur dans le trou dintroduction, et qui puisse ainsi être inséré en douceur dans le trou dintroduction et un ensemble introducteur et un outil médical ayant ledit dilatateur. La présente invention concerne par conséquent un dilatateur (30) ayant une première section (311), une seconde section (312), une troisième section (313), et une quatrième section (314) qui ont différents diamètres et qui sont formées
