A helmet, comprising: a shell that includes a through hole extending from an outer side of the shell to an inner side of the shell; and a grommet (10; 50) arranged on the shell where the through hole is located and forming a vent that draws outside air into the shell, wherein the grommet (10; 50) includes a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole, an operation member rotatably coupled to the base, and a plurality of shutters (13, 14) configured to open and close the opening when the operation member is rotated relative to the base, wherein the opening is a first opening, the operation member is a cover that covers the base and is rotated relative to the base, the plurality of shutters (13, 14) is located between the operation member and the base, and the operation member includes a second opening at a position corresponding to the first opening, characterized in that each of the plurality of shutters (13, 14) includes a guide protrusion (36, 37) projected toward the base, and a driven protrusion (41, 42) projected toward the operation member, the base includes a guide groove (21, 22) engaged with the guide protrusion (36, 37), the operation member includes a cam groove (31, 32) engaged with the driven protrusion (41, 42), and the operation member is configured to move the plurality of shutters (13, 14) in accordance with rotation of the operation member relative to the base. a shell that includes a through hole extending from an outer side of the shell to an inner side of the shell; and a grommet (10; 50) arranged on the shell where the through hole is located and forming a vent that draws outside air into the shell, wherein the grommet (10; 50) includes a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole, an operation member rotatably coupled to the base, and a plurality of shutters (13, 14) configured to open and close the opening when the operation member is rotated relative to the base, a base coupled to the shell where the through hole is located, the base having an opening at a position overlapping the through hole, an operation member rotatably coupled to the base, and a plurality of shutters (13, 14) configured to open and close the opening when the operation member is rotated relative to the base, wherein the opening is a first opening, the operation member is a cover that covers the base and is rotated relative to the base, the plurality of shutters (13, 14) is located between the operation member and the base, and the operation member includes a second opening at a position corresponding to the first opening, each of the plurality of shutters (13, 14) includes a guide protrusion (36, 37) projected toward the base, and a driven protrusion (41, 42) projected toward the operation member, a guide protrusion (36, 37) projected toward the base, and a driven protrusion (41, 42) projected toward the operation member, the base includes a guide groove (21, 22) engaged with the guide protrusion (36, 37), the operation member includes a cam groove (31, 32) engaged with the driven protrusion (41, 42), and the operation member is configured to move the plurality of shutters (13, 14) in accordance with rotation of the operation member relative to the base.