A dilator (1, 2, 3, 5m1 to 5m7) comprising a hollow-shaped shaft (11, 13, 15m1 to 15m5) and a spirally-arranged protruding portion, wherein the shaft (11, 13, 15m1 to 15m5) includes a tapered section (112, 132, 152m4, to 152m7) with an outer diameter of a distal end being smaller than an outer diameter of a proximal end, a distal end section (111, 131) including a proximal end being located at the distal end of the tapered section (112, 132, 152m4, to 152m7) and extending toward a distal end side of the tapered section (112, 132, 152m4, to 152m7) in an axial direction, and a body section (113, 133, 153m4, 153m5) including a distal end being located at the proximal end of the tapered section (112, 132, 152m4, to 152m7) and extending toward a proximal end side of the tapered section (112, 132, 152m4, to 152m7) in the axial direction, the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is provided on each outer peripheral surface of the tapered section (112, 132, 152m4, to 152m7), the distal end section (111, 131), and the body section (113, 133, 153m4, 153m5) and includes gaps between neighboring sections of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) along the axial direction of the shaft (11, 13, 15m1 to 15m5), characterized in that a location being at a distal end side in the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) and including a distal end of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is a first location (211, 221, 231) and a location being at a proximal end side in the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) and at the proximal end side relative to the first location (211, 221, 231) and including a proximal end of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is a second location, and a height of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) at the first location (211, 221, 231) gradually decreases toward the distal end side and/or a height of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) at the second location (212, 222, 232) gradually decreases toward the proximal end side. the shaft (11, 13, 15m1 to 15m5) includes a tapered section (112, 132, 152m4, to 152m7) with an outer diameter of a distal end being smaller than an outer diameter of a proximal end, a distal end section (111, 131) including a proximal end being located at the distal end of the tapered section (112, 132, 152m4, to 152m7) and extending toward a distal end side of the tapered section (112, 132, 152m4, to 152m7) in an axial direction, and a body section (113, 133, 153m4, 153m5) including a distal end being located at the proximal end of the tapered section (112, 132, 152m4, to 152m7) and extending toward a proximal end side of the tapered section (112, 132, 152m4, to 152m7) in the axial direction, the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is provided on each outer peripheral surface of the tapered section (112, 132, 152m4, to 152m7), the distal end section (111, 131), and the body section (113, 133, 153m4, 153m5) and includes gaps between neighboring sections of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) along the axial direction of the shaft (11, 13, 15m1 to 15m5), characterized in that a location being at a distal end side in the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) and including a distal end of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is a first location (211, 221, 231) and a location being at a proximal end side in the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) and at the proximal end side relative to the first location (211, 221, 231) and including a proximal end of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) is a second location, and a height of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) at the first location (211, 221, 231) gradually decreases toward the distal end side and/or a height of the spirally-arranged protruding portion (21 to 23, 25m1 to 25m5) at the second location (212, 222, 232) gradually decreases toward the proximal end side.