To provide a dilator which can suppress rapid change in resistance and can perform smooth insertion into an introduction hole. A dilator 10 includes a distal portion 13, an outer diameter of which increases towards a proximal end side in an axial direction； and a cross-sectional area reduction portion 15 which is provided at least on a largest outer diameter portion 16 of the distal portion, and is obtained by reducing a part of an outer periphery of a circular cross section, and a width W1 of the cross-sectional area reduction portion in a chord direction is equal to or greater than a depth D1 of the cross-sectional area reduction portion in a direction perpendicular to the chord direction.
