A dilator includes a shaft with a tapered portion having an outer diameter that is smaller at a distal end than at a proximal end； a proximal end portion provided at a proximal end side of the tapered portion； and optionally a distal end portion provided at a distal end side of the tapered portion. Pitches of adjacent portions of the spirally-arranged protruding portion provided on the tapered portion are different (either smaller or larger) than those of adjacent portions of the spirally-arranged protruding portion provided on distal end side of the proximal end portion and/or on the distal end portion.
