A dilator (1; 10) comprising: a hollow shaft ; and a grip portion provided on a base end of the shaft , the shaft including: a tapered portion having an outer diameter of a front end smaller than that of a base end; and a base end portion provided at a base end side of the tapered portion and extending to the base end side and a front end portion provided at a front end side of the tapered portion and extending to the front end side; or a base end portion provided at a base end side of the tapered portion and extending to the base end side, without the front end portion , wherein, a spirally-arranged protruding portion is provided on an outer peripheral surface of the shaft , the spirally-arranged protruding portion has gaps between adjacent portions along an axis (A) of the shaft , and in a case where the shaft does not include the front end portion , pitches of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the tapered portion are smaller than those of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the base end portion, and in a case where the shaft includes the front end portion , the pitches of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the tapered portion are smaller than those of adjacent portions along the axis (A) of the spirally-arranged protruding portion (A) provided on the front end portion or the base end portion , characterized in that the spirally-arranged protruding portion includes a coil having a wire wound around on the outer peripheral surface of the shaft . a hollow shaft ; and a grip portion provided on a base end of the shaft , the shaft including: a tapered portion having an outer diameter of a front end smaller than that of a base end; and a base end portion provided at a base end side of the tapered portion and extending to the base end side and a front end portion provided at a front end side of the tapered portion and extending to the front end side; or a base end portion provided at a base end side of the tapered portion and extending to the base end side, without the front end portion , wherein, a tapered portion having an outer diameter of a front end smaller than that of a base end; and a base end portion provided at a base end side of the tapered portion and extending to the base end side and a front end portion provided at a front end side of the tapered portion and extending to the front end side; or a base end portion provided at a base end side of the tapered portion and extending to the base end side, without the front end portion , wherein, a spirally-arranged protruding portion is provided on an outer peripheral surface of the shaft , the spirally-arranged protruding portion has gaps between adjacent portions along an axis (A) of the shaft , and in a case where the shaft does not include the front end portion , pitches of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the tapered portion are smaller than those of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the base end portion, and in a case where the shaft includes the front end portion , the pitches of adjacent portions along the axis (A) of the spirally-arranged protruding portion provided on the tapered portion are smaller than those of adjacent portions along the axis (A) of the spirally-arranged protruding portion (A) provided on the front end portion or the base end portion , characterized in that the spirally-arranged protruding portion includes a coil having a wire wound around on the outer peripheral surface of the shaft . <EOS>