A dilator comprising: a shaft (3; 21; 31) in a hollow shape; and a grip portion that is provided on a proximal end of the shaft (3; 21; 31), wherein the shaft (3; 21; 31) includes: a tapered portion (3D; 24; 34) in which an outer diameter of a distal end is smaller than an outer diameter of a proximal end; and either a distal end portion (3E; 25) that is provided on a distal end side of the tapered portion (24; 34) and which extends toward the distal end side and a proximal end portion (3C; 23; 33) that is provided on a proximal end side of the tapered portion (24; 34) and which extends toward the proximal end side, or the proximal end portion that is provided on the proximal end side of the tapered portion (3D; 24; 34) and which extends toward the proximal end side, without the distal end portion (3E; 25), a spirally-arranged protruding portion (22; 32) is provided on an outer peripheral surface of the shaft (3; 21; 31), the spirally-arranged protruding portion (22; 32) has gaps in sections that are neighboring along an axis of the shaft (3; 21; 31), in a case where the shaft (3; 21; 31) does not include the distal end portion (3E; 25), a pitch (L2; L22; L32) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch (L21; L31) of sections of the spirally-arranged protruding portion (22; 32) provided on the proximal end portion (3C; 23), which are neighboring along the axis, and in a case where the shaft (3; 21; 31) includes the distal end portion (3E; 25), a pitch (L2; L22) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch of sections of the spirally-arranged protruding portion (22; 32) provided on the distal end portion (3E; 25) or the proximal end portion (3C; 23), which are neighboring along the axis. a shaft (3; 21; 31) in a hollow shape; and a grip portion that is provided on a proximal end of the shaft (3; 21; 31), wherein the shaft (3; 21; 31) includes: a tapered portion (3D; 24; 34) in which an outer diameter of a distal end is smaller than an outer diameter of a proximal end; and either a distal end portion (3E; 25) that is provided on a distal end side of the tapered portion (24; 34) and which extends toward the distal end side and a proximal end portion (3C; 23; 33) that is provided on a proximal end side of the tapered portion (24; 34) and which extends toward the proximal end side, or the proximal end portion that is provided on the proximal end side of the tapered portion (3D; 24; 34) and which extends toward the proximal end side, without the distal end portion (3E; 25), a spirally-arranged protruding portion (22; 32) is provided on an outer peripheral surface of the shaft (3; 21; 31), the spirally-arranged protruding portion (22; 32) has gaps in sections that are neighboring along an axis of the shaft (3; 21; 31), in a case where the shaft (3; 21; 31) does not include the distal end portion (3E; 25), a pitch (L2; L22; L32) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch (L21; L31) of sections of the spirally-arranged protruding portion (22; 32) provided on the proximal end portion (3C; 23), which are neighboring along the axis, and in a case where the shaft (3; 21; 31) includes the distal end portion (3E; 25), a pitch (L2; L22) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch of sections of the spirally-arranged protruding portion (22; 32) provided on the distal end portion (3E; 25) or the proximal end portion (3C; 23), which are neighboring along the axis. a tapered portion (3D; 24; 34) in which an outer diameter of a distal end is smaller than an outer diameter of a proximal end; and either a distal end portion (3E; 25) that is provided on a distal end side of the tapered portion (24; 34) and which extends toward the distal end side and a proximal end portion (3C; 23; 33) that is provided on a proximal end side of the tapered portion (24; 34) and which extends toward the proximal end side, or the proximal end portion that is provided on the proximal end side of the tapered portion (3D; 24; 34) and which extends toward the proximal end side, without the distal end portion (3E; 25), a spirally-arranged protruding portion (22; 32) is provided on an outer peripheral surface of the shaft (3; 21; 31), the spirally-arranged protruding portion (22; 32) has gaps in sections that are neighboring along an axis of the shaft (3; 21; 31), in a case where the shaft (3; 21; 31) does not include the distal end portion (3E; 25), a pitch (L2; L22; L32) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch (L21; L31) of sections of the spirally-arranged protruding portion (22; 32) provided on the proximal end portion (3C; 23), which are neighboring along the axis, and in a case where the shaft (3; 21; 31) includes the distal end portion (3E; 25), a pitch (L2; L22) of sections of the spirally-arranged protruding portion (22; 32) provided on the tapered portion (3D; 24; 34), which are neighboring along the axis, is larger than a pitch of sections of the spirally-arranged protruding portion (22; 32) provided on the distal end portion (3E; 25) or the proximal end portion (3C; 23), which are neighboring along the axis.