To provide a dilator which can alleviate diameter expansion resistance sensed by an operator at the time of inserting a dilator into an introduction hole to perform smooth insertion into the introduction hole, and an introducer assembly and a medical tool including such a dilator.A dilator 30 includes a first section 311, a second section 312, a third section 313, and a fourth section 314 which have different diameters from each other and are formed to be adjacent to each other in an axial direction, in that order from a distal end to a proximal end in the axial direction, an angle α1 formed by an outer periphery surface S1 of the first section with respect to the axial direction is smaller than an angle α2 formed by an outer periphery surface S2 of the second section 312 with respect to the axial direction, the angle formed by the outer periphery surface of the second section with respect to the axial direction is larger than an angle α3 formed by an outer periphery surface S3 of the third section 313 with respect to the axial direction, and the angle formed by an outer periphery surface of the third section with respect to the axial direction is smaller than an angle α4 formed by an outer periphery surface S4 of the fourth section 314 with respect to the axial direction.
