A dilator that includes a shaft and a spirally-arranged protruding portion. The shaft includes a tapered section, a distal end section, and a body section. The spirally-arranged protruding portion includes spirally-arranged protruding sections provided on outer peripheral surfaces of each of the tapered section, the distal end section, and the body section, and gaps between adjacent spirally-arranged protruding sections along an axial direction of the shaft. At least one of a height t of the spirally-arranged protruding portion at a first location gradually decreases toward a distal end side and a height of the spirally-arranged protruding portion at a second location gradually decreases toward a proximal end side.