An extensible member (20, 50, 60) comprising: a tubular extensible portion (30, 51, 61) made of a thermoplastic elastomer that has an inner peripheral surface and an outer peripheral surface in a bellows shape in which mountains and troughs are continuous in an axis direction; and at least one protrusion (41, 42, 54, 55, 64) that projects from the outer peripheral surface, wherein the outer peripheral surface includes: outer crests as crests of mountains; outer roots as bottoms of troughs; and first contact surfaces (35, 52, 62) and second contact surfaces (36, 53, 63) between the outer crests and the outer roots that are positioned to be contactable with one another when the extensible portion (30, 51, 61) axially contracts, the inner peripheral surface includes: inner crests as crests of mountains; and inner roots as bottoms of troughs, the inner crests are positioned radially inward of all of the outer roots, and the inner roots are positioned radially inward of all of the outer crests, the protrusion (41, 42, 54, 55, 64) is positioned on at least one of the first contact surfaces (35, 52, 62) and the second contact surfaces (36, 53, 63), and the protrusion includes a plurality of first protrusions (41, 54) that project from the first contact surfaces (35, 52), characterized in that the protrusion further includes a plurality of second protrusions (42, 55) that project from the second contact surfaces (36, 53), and the plurality of first protrusions (41, 54) and the second protrusions (42, 55) are disposed separately in alternation in a circumferential direction on the first contact surfaces (35, 52) and the second contact surfaces (36, 53) contactable with one another. a tubular extensible portion (30, 51, 61) made of a thermoplastic elastomer that has an inner peripheral surface and an outer peripheral surface in a bellows shape in which mountains and troughs are continuous in an axis direction; and at least one protrusion (41, 42, 54, 55, 64) that projects from the outer peripheral surface, wherein the outer peripheral surface includes: outer crests as crests of mountains; outer roots as bottoms of troughs; and first contact surfaces (35, 52, 62) and second contact surfaces (36, 53, 63) between the outer crests and the outer roots that are positioned to be contactable with one another when the extensible portion (30, 51, 61) axially contracts, outer crests as crests of mountains; outer roots as bottoms of troughs; and first contact surfaces (35, 52, 62) and second contact surfaces (36, 53, 63) between the outer crests and the outer roots that are positioned to be contactable with one another when the extensible portion (30, 51, 61) axially contracts, the inner peripheral surface includes: inner crests as crests of mountains; and inner roots as bottoms of troughs, inner crests as crests of mountains; and inner roots as bottoms of troughs, the inner crests are positioned radially inward of all of the outer roots, and the inner roots are positioned radially inward of all of the outer crests, the protrusion (41, 42, 54, 55, 64) is positioned on at least one of the first contact surfaces (35, 52, 62) and the second contact surfaces (36, 53, 63), and the protrusion includes a plurality of first protrusions (41, 54) that project from the first contact surfaces (35, 52), characterized in that the protrusion further includes a plurality of second protrusions (42, 55) that project from the second contact surfaces (36, 53), and the plurality of first protrusions (41, 54) and the second protrusions (42, 55) are disposed separately in alternation in a circumferential direction on the first contact surfaces (35, 52) and the second contact surfaces (36, 53) contactable with one another.